Mechanically toughened EMI shielding natural rubber composite in microwave frequency bands

Abstract

Mechanically toughened high electromagnetic interference (EMI) shielding natural rubber composite material was prepared and characterised. The aim of this investigation was to reveal the role of adding cobalt nanowire and conductive graphene nanoplatelet in toughening and EMI shielding effectiveness of rubber composite in microwave bands (E, F, I and J). The composites were prepared using two roll-mill and cured at 150^οC. The mechanical and EMI shielding behaviour was measured following ASTM standards. The addition of graphene nanoplatelet improved the toughness and skin effect of rubber, whereas cobalt nanowire improved the magnetic permeability. A highest EMI shielding of −17.6 dB in absorption and −20 dB in reflection at 18 GHz was observed for the composite made of graphene nanoplatelet and cobalt nanowire of 3 and 8phr. These mechanically toughened promising EMI shielding composite materials could be used as EMI filters and electromagnetic wave absorbers in antennae application with good toughness.

Keywords

Composite additives mechanical properties dielectric properties magnetic polymer

Introduction

High toughness EMI shielding improved polymer composites are having wider applications in electronics applications particularly in Telecommunication sectors [1]. Usually, the electromagnetic interference (EMI) between gadgets creates a malfunction in signal transmission, data recurring and clarity of data processing [2]. To reduce the EMI between gadgets high-performance flexible shielding materials are getting high attention among rigid thermoset plastic-based materials [3]. In general, elastomers are more flexible and versatile than polymer and metallic materials, thus they are highly suitable in high-performance EMI shielding application. Since elastomers are flexible nature their degree of application and workability is very high in several engineering domains [4]. A lot of researchers studied rigid thermoset plastic-based EMI shielding materials; whereas rubber-based flexible EMI shielding research is marginally less. Moreover, for the past decade, many researchers used conducting materials as filler for improving the shielding effectiveness by absorption (S₂₁) scattering phenomenon but still the shielding efficiency is not so high. But some researchers have been identified that the ferromagnetic selective additions could improve EM shielding. John et al. [5] investigated the role of Fe₃O₄ particle addition along with conducting particle. They concluded that the addition of magnetic particle improved the shielding effect than composite made of TiO₂ particle alone. Tahir et al. [6] studied the EMI effect of adding Fe₃O₄ magnetic fillers along with conducting fillers. The authors concluded that the addition of magnetic filler of significant volume percentage increased the EMI shielding at a higher rate than composite made of only conducting particles. Jin et al. [7] investigated the effect of iron-deposited graphene oxides on the electromagnetic wave absorbing property of polymer composite films with Fe-based hollow magnetic fibres for near-field applications. They concluded that the power losses of the composite films showed that the incorporation of a small amount of Fe-deposited GO into the film alongside the hollow fibres could lead to a dramatic increase in the power loss compared to the other secondary fillers. Similarly, Avanish et al. [8] investigated the phenolic resin-based composite sheets filled with mixtures of reduced graphene oxide, γ-Fe₂O₃ and carbon fibres for excellent EMI shielding in the X-band. The authors concluded that additions of γ-Fe₂O₃ in RGO matrix improved the thermal stability and dielectric loss thereby increasing the shielding effectiveness. Yanhu et al. [9] have done a fabrication of a flexible EMI shielding Fe₃O₄@reduced graphene oxide/natural rubber (NRMG) composite with a segregated network. The author concluded that the presence of Fe₃O₄ enhances the EMI shielding effectiveness (EMI SE) of NRMG composites. The EMI SE value of NRMG composite with 10 phr (part per hundred rubber) rGO is 1.4 times higher than that of NRG composite with the same rGO content in the frequency range of 8.2–12.4 GHz. The specific EMI SE of NRMG composite reaches 26.4 dB mm⁻¹, outperforming the ever-reported polymer/Fe₃O₄@rGO composites with low rGO content. Similarly, Shuiping et al. [10] investigated the flexible PVDF/CNTs/Ni@CNTs composite films possessing excellent EMI shielding and mechanical properties under heat treatment. The unique structure of Ni@CNTs, the flexible PVDF/CNTs/Ni@CNTs composite films exhibit ultrahigh efficient EMI shielding properties with excellent mechanical properties, with an average tensile strength of 70 MPa and a mean elongation at break of 16.9%. Thus it is clear that the addition of magnetic particle could greatly contribute to attenuation of high-frequency electromagnetic waves.

However, the type and shape of reinforcement also could affect the shielding effectiveness. Compare to particle, the nanowire is more effective reinforcement due to their high aspect ratio and continuous structure. Haito et al. [11] experimentally proved that compare to alumina nanoparticle the silica nanowire is much effective in thermo-mechanical and luminance properties. The nanowire absorbed the applied load and transmits it very effectively. Frank et al. [12] investigated the effect of adding alumina nanowire into PEEK and its tribological properties. They concluded that the addition of alumina nanowire produced improved mechanical and wear resistance properties on comparing with composite not filled with alumina nanowire. Thus there are more studies with nanoparticle and their effects in desired properties, but the implication of nanowire and their effects are now slowly emerging. The natural rubber could be selected as matrix since it is a biomaterial and not producing any harmful effect. Graphene nanoplatelet could be selected as conducting material over another allotrope of carbon. It possesses high electrical conductivity up to 900 S/cm [13]. Despite good conductivity, the graphene nanoplatelet does not favor any magnetic effect thus the cobalt nanowire could be selected as a magnetic counterpart. It is a high aspect ratio long continuous structure, having strong magnetism, high susceptibility (ferromagnetic) and high coercivity up to 150–800 G [14]. The hybrid composites could be prepared using two roll-mill since it contains very simple process parameters like no specific temperature requirements, no specific milling speed and normal operating ambience [15]. Wang et al. [16] describe the advantages of the two roll-mill process of making rubber composite. According to their statement in two roll-mill the filler distribution could achieve effectively due to more number of passes. If the dispersed fillers are more uniform in the matrix their targeted outcome would be a great level [17].

Thus based on the previous literature, it is found that the electromagnetic shielding effectiveness based on cobalt nanowire along with graphene nanoplatelet is not yet investigated. Moreover, the research based on the combination of conductive cum magnetic nano fillers in rubber-based composites for EMI shielding also very less. Only fewer numbers of researchers have investigated the effect of adding magnetic fillers along with conductive particles. Moreover, the implication of cobalt nanowire as shielding material also not yet done. Thus the present investigation aimed to scrutinise the EMI shielding effect of graphene nanoplatelet and cobalt nanowire dispersed natural rubber composite and its mechanical properties for being a good shielding material. This high toughness EMI shielding flexible material could be used in telecommunication, radar and mobile networking where high EMI shielding is appreciable.

Materials and methodology

Materials

The natural rubber used in this present study was having a density of 1.5 g/cm³ purchased from Metro composite R&D centre, Chennai, India. In addition to that, the curing agents dibenzothiazole disulphide, tetramethyl thiuram disulfide, zinc oxide and sulphur was purchased from MERCK India, Ltd. Graphene nanoplatelet of the density of 0.2–0.4 g/cm³ with an average particle size of <2 µm, similarly, cobalt nanowire of density 8.94 g/cm³ with diameter 200–300 nm and length of 100–200 µm were purchased from Sigma Aldrich, USA. All materials were used in the as-received condition without any post-process. Figure 1 shows the TEM image of (a) graphene nanoplatelet and (b) cobalt nanowire used in this study.

Composite preparation

Natural rubber shielding composite was prepared using two-roll milling process. The roller speed of 20 rev min⁻¹, nip gap of 3 mm and milling temperature of 30 ^οC (RT) were fixed as process parameters. First the graphene nanoplatelet and cobalt nanowire of the desired phr was dispersed with natural rubber latex and continue milling until getting uniform admix. Second, the milled natural rubber latex then mixed with curing agents MBTS (Dibenzothiazole disulphide), TMT (Tetramethyl thiuram disulfide), sulphur and zinc oxide of 6, 3, 12 and 15 phr. The completely milled composites were cured at 150^οC in a hot oven for 20 min [18]. Table 1 shows the designation of various composites prepared with different composition of materials.

Table 1

Designation of various composites.

Composite	Rubber matrix	Graphene nanoplatelet	Cobalt nanowire
Designation	(phr)	(phr)	(phr)
N	100	-	-
NR₁	100	1	-
NR₂	100	3	-
NR₃	100	1	4
NR₄	100	3	8

Note: NR-Natural rubber

Test specimen preparation

The two roll-mill prepared natural rubber composites were cut for possible testing in-accordance to ASTM standards. The ASTM test specimens were cut using an abrasive water jet machine (Maxium water jets, KENT, USA) with recommended process parameters. Working pressure of 75 psi, an abrasive flow rate of 0.1 g/min, nozzle diameter of 0.9 mm and nib cap of 2 mm was maintained as process parameters [19].

Characterisation

The tensile and tear strength of natural rubber composite was tested using a universal testing machine (INSTRON 5900, UK) followed by ASTM D412 and D624. The micro hardness of composite was examined using a Shore-A durometer (bluesteel, India) following ASTM D 2240 respectively. The hysteresis graph of natural rubber composite was recorded using a vibrato sample magnetometer, having 15,000 G maximum magnetic field (Lakeshore, 7407, USA). The mode of data acquisition was point by point method with 101 nodes. The real and imaginary part of permittivity, permeability and microwave shielding effectiveness was measured via Nicolson–Ross–Weir method using a vector network analyser (Agilent/HP E8362B, 3–18 GHz measuring range). Rectangular specimens of 22 mm length 11 mm width and 3 mm thickness were used as a testing specimen. Equations (1–4) show the formulae of shielding due to wave absorption, wave reflection, wave transmission and total wave shielding effectiveness. (1) (2) (3) (4)

Results and discussions

Mechanical properties

Table 2 shows the tensile, tear and hardness value of the shielding rubber composite. It is observed that the pure natural rubber gives the tensile strength, tear strength and hardness of 35 MPa, 30 N/mm and 41 shore-A. This lower value in mechanical properties is the reason for soft rubber (latex) molecules of natural rubber, which cannot bear the load thus showed the least modulus. There is no crack suppressing mechanisms present in natural rubber thus the crack would grow in the faster manner and produced quicker plastic deformation. But it is noted that further addition of graphene platelet of 1 and 3phr into natural rubber increased the tensile, tear and hardness. Improved tensile strength of 42 and 51 MPa, tear strength of 38 and 44 N/mm and hardness of 43 and 45 Shore-A was observed for composite designations NR₂ and NR₃, respectively. This improvement is because of the presence of graphene nanoplatelet, which could arrest the propagation of micro crack by acting strong hindrance by its plate nature. Thus the crack would not get enough energy to grow further and suppressed along the wall of graphene nanoplatelet. It is observed that on comparing with 1 phr of graphene 3 phr gives improved mechanical properties. Large dispersion of graphene platelet in rubber arrests the micro crack creation and propagation by its large surface area and rectangular plate structure, which is normal to the crack propagation direction, thus increased the mechanical properties [20, 21]. It is observed that further addition of cobalt nanowire of 4 and 8 phr into natural rubber matrix improves the tensile, tear and hardness marginally. A highest tensile strength of 60 MPa, tear strength of 49 N/mm and shore-A hardness of 49 was observed for composite designation NR₄. This improvement is because of the presence of cobalt nanowire of 8 phr and graphene nanoplatelet of 3 phr increase the matrix density and suppresses the crack initiation. The long continuous structured cobalt nanowire improves the load sharing phenomenon in composite thus reduces the stress intensity factor. Lower the stress intensity factor in the in-bound crack mouth led higher the crack suppression thus offer improved tensile and tear strength. Similarly, the hardness improved to the larger scale of 49 shore-A for the composite designation NR₄. This higher hardness is the reason for restriction in stretching of rubber molecule by effective filling and improvement of cross linking density of rubber [22]. Figure 2 shows the TEM image of the fractured portion of natural rubber composite. Figure 2(a) shows the graphene nanoplatelet in natural rubber. The fractured portion holds pit marks and uniform dispersion of graphene platelet, which indicates the improved toughness. Similarly, Figure 2(b) shows the cobalt nanowire in the natural rubber matrix. The nanowire is uniformly dispersed in the matrix and reduces the stress intensity factor, thus improved tensile, tear and hardness [23].

Table 2

Mechanical properties of shielding rubber composite.

Composite	Tensile strength	Tear Strength	Micro hardness
Designation	(MPa)	(N/mm)	(Shore-A)
N	35	30	41
NR1	42	38	43
NR2	51	44	45
NR3	57	46	47
NR4	60	49	49

Dielectric properties

Figure 3 shows the real and imaginary part of the relative permittivity of natural rubber and its composites. It is observed that the relative permittivity of pure rubber gives 2.6 as real part and 2.8 as imaginary part at frequency 3 GHz. The poor polarisation effect is the cause of this lower relative permittivity [24]. It is observed that the further addition of graphene nanoplatelet of 1 and 3 phr increased the relative permittivity. This improvement is the reason for improved conductance of graphene nanoplatelet by continues 2-D carbon structure, which facilitate easy admit of charges. It is further noted that when frequency increases the relative permittivity decreases. This phenomenon is because of the depletion of the polarisation effect due to low relaxation time offers to the rubber dielectric [25]. It is noted that further addition of cobalt nanowire along with graphene nanoplatelet showed very high relative permittivity of 7.2 in real part and 7.5 in the imaginary part. This improved permittivity is the cause of improvement in charge admittance and polarisation effect at a higher frequency. The inclusion of cobalt nanowire facilitates more charge transfer thus higher permittivity is observed. It is noted that the imaginary part measures higher than the real part. This discrimination is the cause of heat loss during higher frequency due to higher amplitude. Moreover, at a higher frequency, the difference between real and imaginary part is high. This phenomenon is the reason for higher heat loss due to more vibration [26].

Magnetic properties

Figure 4 shows the hysteresis behaviour of composites under external magnetic field. It is noted that the addition of 4 and 8 phr of cobalt nanowire increased the total magnetisation, retentivity and coercivity. A higher magnetisation of 940 emu with retentivity of 340 emu was noted for composite designation NR₄. This improvement is the cause of ferromagnetic cobalt nanowire, which increased the alignment of magnetic domains under the external magnetic field. It is noted that the B-H loop of NR₃ is narrowed than NR₄ composite because of soft magnetism whereas in NR₄ the loop is wider and steeper. This phenomenon is the reason for the acquisition of strong magnetism under the external magnetic field. It is noted that the composite designation NR₄ gives more coercivity than NR₃. This increment is the reason for high retained residual magnetism during the forward magnetic field. Thus a large quantity of negative filed is required to bring back the composite to zero magnetic moments [27]. Figure 5 is the graphs of magnetic permeability of natural rubber composites. The ferromagnetic cobalt nanowire increases the magnetic permeability thus higher permeability is observed. A maximum permeability of 2.55 was observed for composite designation NR₄ at 18 GHz (J band). This maximum permeability is the cause of acquisition of magnetic moments at higher frequency with larger rate [28]. It is further noted that the imaginary part of permeability is higher than the real part in all frequency ranges. This difference is the cause of hysteresis losses due to residual magnetism [29]. Thus, the addition of cobalt nanowire improves the magnetic nature of composite to a significant level and improves the hysteresis losses, which in turn suppressing the incoming microwaves not to penetrate in the rubber composite.

Microwave shielding properties

Figure 6 shows the graph of scattering parameters obtained from vector network analyser. It is observed that the pure natural rubber gives poor shielding against microwave frequency due to absorption of 0.2, 0.4, 0.5 and 0.9 dB. These very fewer values are the cause of less conductance of natural rubber. Similarly, the shielding due to reflection is 3.2, 3.8, 4.0 and 4.6 dB. Hence the total shielding measures 3.4, 4.2, 4.5 and 5.5 dB at various microwave frequencies. This poor shielding is the cause of the absence of wave scattering constitutions in natural rubber [30]. It is further observed that the addition of 1 and 3 phr of graphene nanoplatelet into natural rubber increases the wave scattering parameters. The shielding due to absorption for NR₁ composite designation is 2.3, 2.8, 3.3 and 6.4 dB. Similarly, for NR₂ the attenuation was 3.3, 6.4, 11.4 and 14.4 dB. This improvement in absorption shielding mechanism is the cause of improved conductivity of composite due to the presence of graphene nanoplatelet. Since the presence of graphene produced large eddy current when the incoming microwave hit the composite material. This induced eddy current formed a back emf skin, which hinders the penetration of microwaves by attenuating their electrical component [31, 32].

Similar effects were observed in SE _R and SE _T too. The presence of graphene nanoplatelet reflects the penetrated electromagnetic wave multiple times within the shielding composite thus the wave getting weakens and attenuated. The total shielding effect due to the addition of graphene nanoplatelet of 1 and 3 phr is 4.6, 5.4, 5.7, 6.1 dB and 8.3, 9.2, 10.5, 16.7 dB in 3, 4, 10 and 18 GHz. It is noted that when frequency increases the shielding effect also increases. This phenomenon is the cause of very high amplitude microwave produces more crest and trough within shielding composite and loss in their energy level thus larger attenuation is observed [33]. It is noted that further addition of cobalt nanowire of 4 and 8 phr into natural rubber along with graphene nanoplatelet further improves the microwave attenuation. The composite designations NR₃ and NR₄ gives the total shielding effect of 11.1, 19.2, 28, 34.6 dB and 11.5, 18.8, 34, 38.1 dB at various microwave frequencies. This higher attenuation is the cause of the presence of both conducting (graphene) and magnetic (cobalt) fillers, which improves the absorption and reflection shielding mechanisms simultaneously. Since the presence of cobalt nanowire produces high residual magnetism the incoming microwave's magnetic component gets attenuated thus hinders the penetration of waves further [34]. The loss in energy via absorption and multi-reflection dominates more in total shielding effect of the composite. In all frequencies, the magnetic particle added composites alone seen giving larger attenuation than composites contain only conducting material. This effect is because of when conducting particle alone presence the attenuation resulted due to the creation of eddy current, whereas composite contains both conducting and magnetic fillers the attenuation resulted due to eddy current and also high residual magnetism [35]. Thus the addition of both graphene and cobalt nanowire into natural rubber showed the improvement of 60, 71, 83, and 84% in total shielding effect at 3 GHz (E band), 4 GHz (F band), 10 GHz (I band) and 18 GHz (J band) microwave frequencies.

Conclusions

High-performance natural rubber shielding material was prepared using graphene nanoplatelet and cobalt nanowire. The shielding composites were prepared using two-roll mill process followed by curing. The shielding composites are further subjected to microwave shielding test to reveals the effectiveness of magnetic filler addition in total shielding effect of rubber composite. The detailed summary of the project as follows.

The mechanical results show that the addition of graphene nanoplatelet and cobalt nanowire into natural rubber composite increases the tensile and tear strength with improved hardness.

The transmission electron microscope image of both graphene nanoplatelet and cobalt nanowire showed uniform dispersion in the matrix, which produces improved mechanical properties.

The real and imaginary part of relative permittivity shows improvement in filler addition. The relative permittivity is increased with an increase of filler content however, the relative permittivity decreased with the increase of frequency.

The magnetic properties of cobalt nanowire dispersed natural rubber composite shows improved magnetisation. The 8 phr of cobalt nanowire in natural rubber composite shows the highest magnetisation.

The microwave shielding behaviour of natural rubber composite showed improved attenuation against microwave penetration. The composite designation NR₄ giving higher attenuation of 38.01 dB in 18 GHz (J band). The graphene nanoplatelet of 3 phr and cobalt nanowire of 8 phr was found to be producing the highest wave attenuation.

Thus the inclusion of ferromagnetic cobalt nanowire gives significant contribution in the dielectric, magnetic and microwave attenuation behaviour of composite than composite contains only conductive particle. Moreover, the addition of graphene nanoplatelet and cobalt nanowire significantly improved the mechanical properties also. Thus the shielding composites are mechanically tougher and good in EMI shielding too.

These mechanically toughened microwave shielding ability flexible rubber composites could be used as a shielding material for electronic gadgets, radar, telecommunication cables and microwave shielding helmets.

Conflict of interest

All the authors hereby confirmed that there is no conflict of interest.

Footnotes

Acknowledgement

The authors of this work deeply acknowledge ‘Metro composites research and development centre’, Chennai, India, for the supports in fabrication and testing of composite materials.

Disclosure statement

No potential conflict of interest was reported by the author(s).

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