Microwave properties of natural rubber based composites containing carbon black-magnetite hybrid fillers

2.1 Materials

Natural rubber (NR) (SMR 10) was purchased from North Special Rubber Corporation of Hengshui, Hebei Province, China. Other ingredients such as zinc oxide (ZnO), stearic acid (SA), N-tert-butyl-2-benzothiazyl sulfenamide (TBBS) (Vulkacit NZ/EG, produced by Lanxess), and sulphur (S) were commercial grades and used without further purification.

Carbon black (Corax N 330, produced by Evonik) had particle diameter of 38–42 nm, specific surface area of 75–85 m²/g, oil absorption number of 95–105 ml/100 g, and density of 1.86 g/cm³. Magnetite containing 95–100% of Fe₃O₄ and 0–5% of silica, with particle size of −50 microns, was purchased from Inoxia, UK.

2.2 Carbon black-magnetite hybrid filler preparation

Fifty grams of carbon black and 50 of magnetite were loaded into a ball mill, and 500 ml of ethyl alcohol was poured over it. The mixture was homogenized for 2 h. The suspension thus obtained was dried at 50°C for 1 h. Then the temperature was raised to 200°C, and the drying continued for 2 more hours. The yield was ground in a ball mill for 2 h. After that, the grind was loaded into the reactor (shown in Figure 1) and heated at 440°C under 10⁻² mm Hg vacuum for 2 h. If necessary, the product was ground once more in a ball mill.

Figure 1:

Design of the reactor used: 1 – reactor vessel, 2 – top (sealing flange), 3 – sealing nuts, 4 – gas inlet, 5 – gas outlet, 6 – thermocouple, 7 – Buchner funnel.

2.3 Carbon black-magnetite hybrid filler characterization

The following methods were used to characterize the filler:

• iodine adsorption number – in correspondence to that of Ref. [21],

• oil absorption number – in correspondence to that of Ref. [22],

• specific surface area – calculated according to BET method [23], and

• mesopore volume – calculated by Gurvich rule [24], [25], [26].

The average diameter of the mesopores and their size distribution were determined by the method of Barett-Joyner-Halenda (BJH) [27]. The texture characteristics were determined by low-temperature (77.4 K) nitrogen adsorption on a Quanta chrome Instruments NOVA 1200e (USA) apparatus. The nitrogen adsorption-desorption isotherms were analyzed to evaluate the following parameters: the specific surface areas (SBET) were determined on the basis of the BET equation, and the total pore volume (Vt) was estimated in accordance with the Gurvich rule at a relative pressure close to 0.99. Pore size distribution was calculated using the BJH method. The ash content of the carbon-magnetite filler was determined according to Ref. [28]. The ash from the filler was studied by complete silicate analysis, weight analysis, atomic absorption spectroscopy (AAS) (Perkin Elmer 5000, Perkin Elmer, Waltham, MA, USA), and inductively coupled plasma-optical emission spectroscopy (ICP-OES) (“Prodigy” High Dispersion ICP-OES, Teledyne Lemas Labs). Powder X-ray diffraction (XRD) patterns were collected at room temperature on a Bruker D8 (Billerica, MA, USA) Advance diffractometer using CuKα radiation and a LynxEye position sensitive detector (PSD) within the 5.3–80° 2θ range, step 0.04° 2θ, and 0.1 s/strip (total of 17.5 s/step) counting time. To improve the statistics, sample rotating speed of 30 rpm was used. Crystalline phases were identified by a DiffracPlus EVA 12 program (Bruker software) and International Center for Diffraction Data (ICDD) PDF-2 (2009) database. For ascertaining the distribution of carbon black and magnetite, the hybrid products were also investigated by scanning transmission electron microscope (STEM). Transmission electron microscope (TEM) investigations were performed on a TEM JEOL 2100 (Jeol USA, Inc.) instrument at accelerating voltage of 200 kV. The specimens were prepared by grinding and dispersing them in ethanol via ultrasonic treatment for 6 min. The suspensions were dripped on standard holey carbon/Cu grids (TED PELLA, Inc., USA). The measurements of lattice-fringe spacing recorded in high-resolution transmission electron microscopy (HRTEM) micrographs were made using digital image analysis of reciprocal space parameters. The analysis was carried out by Digital Micrograph software: TEM JEOL 2100; energy-dispersive X-ray spectroscopy (XEDS): Oxford Instruments, X-Max^N 80T; charge coupled device (CCD) Camera Orius 1000, 11 Mp, Gatan Inc. The ratio of magnetite to carbon counts was used as an estimate of the relative ranking of the amount of the two components in certain domains in the filler aggregates.

2.4 Preparation of composites containing carbon black-magnetite hybrid filler

The formulations of the NR-based compounds, containing a carbon-magnetite hybrid filler or the physical mixture of the same components, are shown in Table 1.

Elastomer compounds were prepared on an open laboratory two rolls mill with rolls dimensions of L/D 320×160 and 1.27 friction. The turns of slowly rotating roll were 25 min⁻¹. Test samples were vulcanized to plates with dimensions of 6 and 8 cm in vulcanization optimum, determined according to their vulcanization isotherms (ISO 3417:2010) at 160°C in an electrically heated vulcanization hydraulic press at 10 MPa on a steel made press form.

2.5 Characterization of the rubber based composites containing hybrid fillers

2.5.2 Resistivity measurement

Volume resistivity (ρ_v, Ω·m) of the obtained flat rubber-based samples was measured using two electrodes (two-terminal method) and calculated by the equation:

(1)ρv= Rv⋅S/h,

where:

• R_v – measured ohmic resistance between the electrodes, Ω;

• S – cross sectional area of the measuring electrode, m²;

• h – sample thickness between the electrodes, m.

The resistance was measured on a teraohmmeter Teralin III (produced in Germany), using direct current (Figure 2).

Figure 2:

Scheme of the laboratory equipment for volume resistivity measurements.

The current voltage was 100 V. The resistance values were measured 1 min after switching on the current, when the resistance values were already stabilized. The measurements were carried out at ambient temperature. The samples used were circles with a diameter of 90 mm and thickness of 2 mm.

2.6 Microwave properties measurement

2.6.1 Reflection and attenuation

Measurements of reflection and attenuation were carried out using the measurement of output (adopted) power P_a in the output of a measuring line without losses, where samples of materials may be included. Because of the wide frequency measurement, a coaxial line was used. Samples of the materials were shaped into discs with an external diameter D=20.6 mm, equal to the outer diameter of the coaxial line and thickness of Δ≈2 mm. The internal diameter depended on the relative dielectric permittivity of the material.

The sample reflected a part of the incident electromagnetic wave with power P_in. The rest of the wave with power P_p penetrated the material, so that the attenuation L depended on the coefficient of reflection |Γ|. Its module was determined by a reflect meter.

Thus, the attenuation was determined by

(2)L=10 logPaPp, dB,

where

(3)Pp=Pin.(1−|Γ|2)

The following scheme presents the equipment used for testing both parameters (Figure 3).

Figure 3:

Scheme of the equipment for measuring the microwave properties. 1 – A set of generators for the whole range: HP686A and G4 – 79 to 82; 2 – coaxial section of the deck E2M Orion, with samples of material; 3 – power meter HP432A; 4 – scalar reflectance meter HP416A; R – reflect meter, including two directional couplers Narda 4222.16; two crystal detectors Narda 4503-N.

3.1 Carbon black-magnetite hybrid filler characterization

Table 2 summarizes some properties of the obtained hybrid filler characterizing its specific features.

Noteworthy is that the oil absorption number of the hybrid filler is lower if compared to the one of neat carbon black used as a substrate (95–105 ml/100 g). This evidences that the distribution of magnetite phase over the carbon one is what lessens its proneness to form secondary structures (aggregates and agglomerates). As it will be shown below, the effect benefits the microwave properties of the composite. The results from the analyses (silicate analysis, AAS, and ICP-OES) of the filler ashes are presented in Table 3.

As expected, the ashes contain mostly magnetite and silica, which have not undergone changes because of the temperature at which the ash content is determined. It is also seen that the initial carbon-magnetite ratio has not changed (Table 2). That fact is also important.

The texture analysis and texture characteristics of the obtained hybrid filler are plotted in Figures 4 and 5.

Figure 4:

Dependence of adsorbed gas volume on cm³/g, on relative pressure p/p⁰ (black line – adsorption branch of the sorption isotherm; red line – desorption branch).

Figure 5:

Dependence of derivative volume dV on pore size, nm.

The broad hysteresis in the saturation zone (Figure 4) shows that the pores in the hybrid filler are a secondary formation resultant from the aggregation of the two phases. On the other hand, the narrow hysteresis allows the supposition that the pores are intra-aggregate ones and quite few. That is also confirmed by the reversible desorption branch of the sorption isotherm. The mesopore size distribution curve in Figure 5 shows that the size of the intra-aggregate pores varies in a large range (except for those up to 7 nm). They result from the interpenetration and aggregation of magnetite and carbon black, while the pores of smaller diameter are most probably entirely carbon black aggregates.

It is doubtless that energy dispersive X-ray (EDX) spectroscopy using a STEM or STEM-EDX is the most appropriate research approach to the characterization of hybrid fillers phase. The high angle annular dark field (HAADF) in STEM imaging displays the compositional contrast resulting from elements of different atomic number and their distribution. EDX allows identifying the particular elements and their relative proportions. Initial EDX analysis usually involves the generation of an X-ray spectrum from the entire scan area of the STEM image. The Y-axis shows the counts (number of X-rays received and processed by the detector), and the X-axis shows the energy of those X-rays. In addition, EDX software can:

• keep the electron beam stationary on a spot or on a series of spots and generate spectra that will provide more localized elemental information,

• have the electron beam follow a line drawn on the sample image and generate a plot of the relative proportions of different elements along that line (one-dimensional line scanning), and

• map the distribution and relative proportion (intensity) of different elements over the scanned area (two-dimensional mapping).

Modern software now collects entire cumulative spectra from each point, so element choices can be made post-acquisition.

The representative HAADF image and compositional maps of the carbon black-magnetite hybrid filler are shown in Figure 6. The bright field TEM images of the same sample are shown in Figure 7.

Figure 6:

HAADF image (A) and compositional maps (B, C, D) of the carbon black-magnetite hybrid filler.

Figure 7:

Bright field TEM images at different magnifications (A–D). (A) ×10,000; (B) ×25,000; (C) ×250,000; (D) ×500,000.

As the images in Figures 6 and 7 show, the magnetite phase is evenly distributed among the carbon black agglomerates, in some cases, penetrating them. The latter fact is proven by the high-resolution TEM micrographs (Figure 7C and D), which picture how the magnetite phase penetration had disturbed the structural orderliness and homogeneity of the layers building the elemental particles of carbon black.

The areas where energy dispersive spectra on STEM of the carbon black-magnetite filler were scanned are shown in Figures 8 and 9. Table 4 presents the content of elements carbon, oxygen, and iron in atom percents.

Figure 8:

EDS on STEM of carbon-magnetite hybrid filler: spectra positions (136–144).

Figure 9:

EDS on STEM of carbon-magnetite hybrid filler: spectra positions (145–148).

The analysis in Figures 8 and 9 and the comparison of the spectral data in Table 4 confirm that the magnetite phase (the darker one) is distributed both among the carbon particles and in the carbon aggregates. With some exceptions (spectra nos. 139, 142, 144, 148), the carbon phase is evenly distributed and is visibly present in the spectra.

It has been also important to establish the difference between the behavior of the hybrid filler and physical mixture of its components upon X-ray irradiation. The X-ray diffractograms of the fillers (Figures 10 and 11, respectively) indicate such a difference in the form of texture changes in the hybrid filler caused by thermal activation during its synthesis. The thermal treatment yields a more pronounced texture, but new phases are not formed.

Figure 10:

X-ray diffraction pattern of the obtained hybrid filler.

Figure 11:

X-ray diffraction pattern of the physical mixture carbon black-magnetite.

3.2 Mechanical properties

The mechanical properties of the investigated NR composites are summarized in Table 5. The vulcanizates comprising hybrid filler have Modulus 100 (M₁₀₀) and tensile strength values higher than those of the vulcanizates comprising the physical mixture of carbon black and magnetite. Elongation at break of NR composites containing the hybrid filler is lower than that for the composites containing physical mixture of carbon black and magnetite.

The table also shows the advantage of the dual-phase filler to the physical mixture of the components at the same ratio. Obviously, the thermal treatment at 440°C under vacuum to which the hybrid filler has been subjected favors its texture characteristics, mainly its micropores/mesopores ration and its mesopores size. The larger mesopores of the hybrid filler (Figure 5) facilitate enhanced elastomer-filler interaction, which in turn explains the achieved better physicomechanical properties.

3.3 Resistivity measurement

The data from the measurements on the specific volume resistance and surface resistance of composites comprising the hybrid carbon black-magnetite filler and of composites filled with the physical mixture of carbon black and magnetite are presented in Table 6.

The considerable difference in the specific volume resistance and surface resistance of the composites is obvious in Table 6. Carbon black conductivity is higher than that of magnetite [20], and the data in the table confirm the supposition that the latter is distributed intra-aggregately and inter-aggregately with regard to the carbon phase and limits the contacts between its electroconductive structures. As a result, the total electrical conductivity of the hybrid filler decreases and the specific surface resistance increases, respectively.

3.4 Microwave properties measurement

Figures 12–14 summarize the microwave properties of the composites studied within the 1–12-GHz frequency range.

Figure 12:

Frequency dependence of reflection coefficients |Γ| of a composite containing a carbon black-magnetite hybrid filler (S 1) and a composite containing a physical mixture (S 2).

Figure 13:

Frequency dependence of attenuation coefficients of electromagnetic waves α of a composite containing carbon black-magnetite hybrid filler (S 1) and of a composite containing the physical mixture (S 2).

Figure 14:

Frequency dependence of shielding effectiveness, SE, of a composite containing carbon black-magnetite hybrid filler (S 1) and a composite containing their physical mixture (S 2).

The coefficients of reflection of a composite containing carbon black-magnetite hybrid filler (denoted as S 1) and of a composite containing the physical mixture (denoted as S 2) depending on frequency are shown in Figure 12.

The attenuation coefficients of a composite containing carbon black-magnetite hybrid filler (S1) and of a composite containing the physical mixture (S 2) depending on frequency are shown in Figure 13.

As seen in Figure 12, the values of the reflection coefficient of both composites increase with the increasing frequency. The increase in the values of composite S 1 is from 0.075 to 0.22, while in those of composite S 2, it is from 0.15 to 0.65. The increase in the values of the attenuation coefficient (Figure 13) with the increasing frequency is more pronounced. That concerns a rather large frequency range (1–12 GHz) and, first of all, frequencies over 6 GHz. The attenuation coefficient values over 30 dB/cm in the 8–12-GHz frequency range allow the statement that the NR-based composite we have prepared and the hybrid carbon black-magnetite filler do possess microwave absorption properties.

The EMI of a composite containing carbon black-magnetite hybrid filler and a composite containing physical mixture depending on frequency is shown in Figure 14.

The composite containing the physical mixture has rather low SE values (between 1.02 and 6.32), while the one containing the hybrid filler having SE values between 10 and 22 dB facilitates effective shielding. The results show that the composite containing carbon black-magnetite hybrid filler exhibits exceptional broadband properties (practically throughout the entire investigated range) in terms of the effectiveness of electromagnetic shielding.