Effect of Alumina Modified by Silane on the Mechanical, Swelling and Dielectric Properties of Al2O3/EPDM/SBR Blend Composites

Rubber blending has been widely used to improve various properties in finished rubber products. The purpose of this paper is to investigate the effect of filler size, type, concentration, and surface treatment on the mechanical, swelling, dielectric, and morphology properties of ethylene propylene diene rubber (EPDM)/styrene-butadiene rubber (SBR) rubber blend nanocomposites filled with alumina (Al2O3) nanoparticles. Bis-(3-(triethoxysilyl)-propyl)-tetrasulfide (TESPT) was used as a silane coupling agent. Rubber blend nanocomposites containing Al2O3 nanoparticles were compared to carbon black (CB). The results indicate that the nanocomposites prepared, in particular with the TESPT, had better properties than the composites without the TESPT. Mechanical properties of the CB-filled EPDM/SBR rubber blend were noticeably improved, indicating CB's inherent reinforcing potential. We found that rubber nanocomposites' crosslink density and filler–rubber interaction increase, and the swelling coefficient decreases with increasing filler content. The dielectric properties of the nanocomposites showed a significant enhancement with fillers. SEM images showed that the CB sample had a higher distribution than Al2O3 due to good interaction and dispersion. This study's experimental data will help design and manufacture outdoor insulators.


Introduction
In the early days, outdoor insulators were made with glass or porcelain due to their preferable mechanical and electrical properties and ability to withstand long-term ageing. For many decades, glass and porcelain were the preferred materials for terminations, housing insulators, surge arrestors, and bushings. Recently, polymeric insulators have been developed, and the improvements in design and manufacturing have made them more and more attractive for utilities. Moreover, they have better dielectric properties, are lightweight, and have low costs than porcelain or glass insulators. On the other hand, they have disadvantages such as lower dielectric constant and thermal stability than ceramic materials. Many studies on their dielectric properties under contamination conditions have been conducted. Through suitable doping, many scientific research studies have enhanced polymers' optical and electrical properties [1][2][3].
Polymers, such as EPDM, are popular polymeric components for electrical insulators. Notably, EPDM rubber materials have been expanded in wires, noise isolation, vibration, tire sidewalls, outdoor insulation system products, and cables due to their low cost, high elongation, thermal resilience, lightweight, and improved wear resistance. It also has excellent temperature, ozone, and oxygen resistance. However, due to its low rigidity, EPDM rubber cannot be used for primary structural applications without adding fillers such as expanded graphite (EG) or nanoparticles of Al 2 O 3 [3,4]. SBR is now widely used for sheathing and insulation because it is more cost-effective than natural rubber. It has excellent properties, such as heat ageing, processability, and 1 3 elasticity. However, because SBR has poor tensile strength and flame retardancy, choosing a suitable filler is critical [5].
Blending elastomers is an effective method for significantly improving material performance to meet industry needs and scientific challenges for a specific set of properties [6]. Because of their unstable morphologies, most blends are immiscible and have poor interfacial properties. Compatibility is an essential tool for such rubber blends. Compatibilizer is added to the blend system to reduce coalescence and interfacial tension, resulting in better coalescence stability and finer phase dispersion [7,8]. The blending of EPDM and SBR allows producing a vulcanizate with the best properties from each component. It also aimed to determine the percolation threshold of such systems concerning the network formation when loaded with different fillers in increasing quantities.
Most particulate fillers are inorganic, and their surfaces are incompatible with polymers. Surface modification of fillers is an excellent method for improving their dispersive properties and surface activity and is thus widely used in preparing rubber composites. Silane coupling agents improve filler dispersion by reducing filler-filler interactions and increasing the filler's compatibility with the rubber [9][10][11].
The mechanical and dielectric properties of insulating rubbers can be enhanced by introducing fillers such as carbon black [12,13] and alumina nanoparticles [14,15] in the backbone of the polymer chain [16]. The electrical properties of various rubbers and their blends have been investigated. It has been shown that the dielectric properties of rubber and rubber blends depend, in general, on the morphology, crystallinity, structure, and presence of other additives [3,7,17]. Phase separation in rubber blends can be detected using light scattering, DSC, and dielectric relaxation methods. The dielectric technique can measure the dielectric properties over various frequencies [16][17][18].
Many investigators have reported the mechanical and electrical behavior of polymer composites filled with different fillers [3,5,7,19]. Nanocomposites studied nano clay's morphology and dielectric properties incorporated in silicone rubber (SR) and EPDM nanocomposites. The results showed that the polar groups of the clay particles improved the nanocomposites' polarisation capability [3]. Nanocomposites' microstructure and dielectric properties based on SBR with varying amounts of nanoparticles of manganese tungstate (MnWO 4 ) were investigated. The FTIR, SEM, and XRD analysis revealed a clear interaction between the nanoparticles and a uniform distribution of nanoparticles in the SBR rubber matrix. Nanoparticles improved the glass transition temperature, dielectric properties, and thermal stability of composites [5].
Cibi Komalan et al. [7] investigated the effect of compatibilization and blend ratio on the morphological, dielectric performance, and electrical of nylon copolymer (PA6, 66)/EPDM rubber blends. Blend compatibilization increased the dielectric constant of the rubber blend vulcanized. The addition of 2.5% of compatibilizer gave the highest dielectric constant value. A conventional mill mixing method was used to investigate the effect of the blend ratio on the mechanical properties, morphology, AC conductivity, and dielectric properties of chlorinated nitrile rubber (Cl-NBR) that had been blended with chlorinated ethylene propylene diene rubber (Cl-EPDM) [19]. According to SEM images, Cl-NBR and Cl-EPDM were mixed uniformly in a 50/50 rubber blend ratio. The polymer blend's elongation at break decreases as the Cl-NBR content increases, whereas the oil resistance increases as the Cl-NBR content increases. A polymer blend's AC conductivity and dielectric properties increased as the Cl-NBR ratio in the blend increased.
To our knowledge, no work has been reported on the mechanical, swelling, and thermal properties of Al 2 O 3 / EPDM/SBR rubber blend composites. Therefore, this study aimed to investigate the effect of incorporating nano Al 2 O 3 in rubber blend composites. The resulting rubber blend nanocomposites' was also evaluated in their mechanical, swelling, dielectric, and morphology properties.

Materials
EPDM Vistalon 650S, diene (ethylene nonbornene) content of 9%, ethylene content of 55%, Mooney viscosity ML (1 + 8) of 48-52 at 127 °C, and specific gravity of 0.86, was produced by ESSO Chemi, Germany. SBR (1502), styrene content of 23.5%, Mooney viscosity ML (1 + 4) of 48-58 at 100 °C, and specific gravity of 0.945 and carbon black (CB) with an average size of 28 nm were purchased from Marceleno Company for Chemical Industry and Trade, Cairo, Egypt. Al 2 O 3 nanoparticles with an average size of 20 -50 nm were purchased by Nano-Tech Egypt, Egypt. Al 2 O 3 particles with an average size of 120 -320 nm were prepared by ball milling. Bis(3-triethoxy silyl propyl) tetrasulfide (TESPT) was purchased by Sigma-Aldrich, and it was used as a chemical coupling agent. N, N′-methylene bisacrylamide was used as a crosslinker and purchased from MERCK-Schucharai Bei Munchen. Cyclohexyl benzothiazole sulphenamide (CBS) was used as an accelerator and supplied from Rheinehemie Germany, and Polymerized 2,2,4-trimethyl-1,2-dihydroquinoline (TMQ) was used as an antioxidant is commercial grade product. Maleic anhydride (MAH) has M w = 98.06 g/mol, a melting point of 54-56 °C, and a specific gravity of 1.48 was purchased from Aldrich Company, Germany. Activators ( Zinc oxide and stearic acid) with specific gravity at 15 °C of 5.55-5.61 and 0.90-0.97, respectively.

Surface Modification of Al 2 O 3 Particles by Silane
Coupling Agents Thongsang

Preparation of Rubber Blend Nanocomposites
The fillers-filled rubber blends were mixed on a laboratory two-roll mill of 470 mm diameter and 360 mm working distance. The speed of the slow roller was 24 rpm, with a 1:1.4 gear ratio according to ASTM D 3184-89 [21]. The formulations are given in Table 1. The curing time was determined by a Monsanto Moving Die Rheometer (MDR 100) according to ASTM D 2084 [22]. The prepared rubber blend composites compounds were vulcanized at 125 bar and 150 °C cure temperature to obtain rubber sheets.

Tensile Properties
Dumbbell shape samples prepared according to ASTM D 412-16 [23] were used to determine the tensile properties of the rubber blend nanocomposites. The tensile strength and elongation at break (%) of the rubber vulcanizates were determined with a Zwick/Roell Z010 testing machine (Germany).

Swelling Properties
Swelling properties were determined according to ASTM D3616 [24], where swelling of the specimens pieces was determined in toluene (solvent). Each specimen was weighed in a weighing bottle and covered with toluene for 24 h to achieve equilibrium swelling. The swollen samples were weighed before being dried to a constant weight in an oven. The final weight was taken as the correct weight of the dissolved matterfree sample. The equilibrium swelling ratio (Q%) is as follows in Eq. 1.
where M c is the molar mass between crosslinks, ρ p is the rubber density (EPDM = 1.1 g/cm 3 and SBR = 0.913 g/cm 3 ), V s is the molar volume of the solvent = 106.35 cm 3 /mol, V r is the volume fraction of swollen rubber, Q is the swelling mass of rubber filled with alumina in toluene. At the same time, (χ) is the interaction parameter between the rubber network and solvent (EPDM = 0.393 and SBR = 0.446). To study the rubber-filler interaction, the Lorenz and Park equations Eq. 5 were applied [26]: where Q filler is the swelling value of the filler and Q gum is the swelling value of the gum.

Dielectric Spectroscopy
The permittivity ε′ and the dielectric loss ε′′ of filler-filled EPDM/SBR rubber blend nanocomposites were measured using an impedance analyzer (Schlumberger Solartron 1260), an electrometer, an amplifier, and measuring cell over a room temperature ~ 30 °C and at frequencies of 0.1 k Hz, 1 kHz, and 10 kHz. The errors in and amount to 1% and 3%, x 100 respectively. A temperature regulator with a Pt 100 sensor was used to regulate the temperature of the samples. Temperature measurements have an error of 0.5 °C. To avoid moisture, the samples were stored in desiccators containing silica gel. The sample was then transferred to the measuring cell and left with P 2 O 5 until the measurements were completed, as described before [27].

Morphology of Rubber Blend Nanocomposites
Fourier-transform infrared (FTIR) spectroscopy, JAS-COFTIR-6000 E (Japan), operated in the absorption mode was used to determine the functional groups in the samples. The analysis was conducted at wavelengths between 400 and 4000 cm −1 at room temperature, and the KBr tablet method was employed to evaluate the functional groups of the samples by mixing with KBr (potassium bromide) disks. The spectra were obtained with a resolution of 4 cm −1 with Model ATR PRO450-S. Morphologies of the rubber from tensile fracture surfaces were studied using scanning electron microscopy (SEM) (JEOL, JSM 6360LA, Tokyo, Japan) at 25 kV accelerating voltage.

FTIR Spectroscopy
The FTIR spectroscopy was used to confirm that the Al 2 O 3 particles had been properly treated with the silane coupling agent. Figure 1 shows the FTIR spectra of untreated and treated Al 2 O 3 particles. The grafting of the silane coupling agent on the surface of the particles resulted in the appearance of several new bands that were not present in the untreated one, in addition to the bands characteristic of Al 2 O 3 particles. For example, the stretching mode of surface hydroxyl groups was responsible for the broad peak around 3420.29 and 3393.53 cm −1 for micro-and nano-Al 2 O 3 particles, respectively, as shown in Fig. 1

Tensile Properties
The tensile properties of the filler/EPDM/SBR rubber blend nanocomposites are presented in Fig. 2. The tensile strength and elongation at break (%) of the nanoparticles Al 2 O 3 filled rubber blend were compared with/without surface treatment by TESPT and crosslinker. Figure 2 clearly shows that the properties of the treated Al 2 O 3 particles-filled vulcanizates with MAH were higher than those of the untreated Al 2 O 3 particles-filled vulcanizates and treated Al 2 O 3 particles-filled vulcanizates without MAH, showing the significant effect of Al 2 O 3 particles surface treatment on improving the tensile properties of Al 2 O 3 particle-reinforced vulcanizates.
The addition of a silane coupling agent increased the tensile strength of the composites to a greater extent, as shown in Fig. 2. The results also revealed that the composites containing TESPT had higher tensile strength than those without it. A silane binding agent improved rubber-filler interactions while decreasing filler-filler interactions in EPDM/ SBR rubber blend nanocomposites, as shown in Fig. 7. Furthermore, the organofunctional group of the organosilane provides additional crosslinks. These effects may have contributed to the improved tensile properties of the treated Al 2 O 3 composites over the untreated alumina composites, as evidenced by the stress-strain study [30]. However, when using MAH as a compatibilizer, the tensile strength of the composites increased with the favorable loading of Al 2 O 3 [8]. These results agree with those of a previous study [14]. The tensile strength of the samples contains 10 phr (C1, M3, and N2), and carbon black was higher than Al 2 O 3 due to good interaction and dispersion. Compared to Al 2 O 3 particles filler, carbon black filler has a more substantial reinforcing effect [31]. Also, the nanoparticles of Al 2 O 3 were higher than the microparticles of Al 2 O 3 (e.g., N2 and M3) due to good interaction and dispersion. Figure 2 also depicts the elongation at break (%) of EPDM/SBR rubber blend nanocomposites with and without a silane coupling agent. With increasing Al 2 O 3 particle loading, the elongation at break (%) increased. The increase in elongation at break (%) of the samples (e.g., M2 and M4) indicates good interfacial adhesion between the rubber and the filler and proper Al 2 O 3 wetting. In addition, the elongation at break (%) of the samples that used MAH as a compatibilizer was higher than those that used crosslinker. The reduction in the elongation at break (%) of the samples (e.g., M2 and M5) indicates the restriction of movement in polymeric molecular chains [32]. The addition of 25 phr of microparticles Al 2 O 3 with TESPT to neat EPDM/SBR rubber blend resulted in a roughly 66% increase in tensile strength, from 1.57 MPa to 2.62 MPa, and a 108% increase in elongation at break (%), from 234% to 486.86%, as shown in Fig. 2. From the tensile strength analysis, the presence of TESPT can increase the dispersion capability of Al 2 O 3 particles and their interfacial interaction with the EPDM/ SBR rubber blend.

Swelling Properties
The variation in crosslink density of the EPDM/SBR rubber blend in the presence of carbon black and Al 2 O 3 particles with/without TESPT is represented in Tables 2, 3 and 4. These tables show that EPDM/SBR rubber blend nanocomposites (e.g., B0, M1, and M2) increase with an increasing    alumina concentration in the matrix. The crosslinking density of the composites with TESPT (M4 and M5) was higher than that of the nanocomposites without TESPT (B0). This was due to the rise in the crosslink density of the rubber composite [14]. Also, the value of crosslink density was even higher for EPDM/SBR rubber blend containing silanetreated nanoparticles Al 2 O 3 with crosslinker compared to samples with untreated nanoparticles Al 2 O 3 and treated particles Al 2 O 3 with MAH. A higher (Q filler /Q gum ) ratio can reduce the interaction of the filler with the rubber matrix. As a result, the swelling ratio for composites containing nanoparticles Al 2 O 3 was significantly lower than for the blank EPDM/SBR rubber blend. As previously explained, the low swelling ratio of nanoparticles Al 2 O 3 composites compared to blank EPDM/SBR rubber blend is most likely due to the higher cure degree indicated by the high torque values. As a result, adding nanoparticles of Al 2 O 3 to the EPDM/ SBR rubber blend matrix reduces the swelling-induced extensibility of the rubber chains. This makes it difficult for the solvent to penetrate the gaps between the rubber molecules, resulting in lower swelling. The lower swelling ratio indicates that the polymer and toluene have a lower interaction. As shown in Tables 2, 3, and 4, increasing the  crosslinking density reduces the molecular movement of the rubber, making it more difficult for the solvent to penetrate. This could also be due to the interaction between the EPDM/SBR rubber blend matrix and Al 2 O 3 , increasing the crosslinking rate. Q filler /Q gum decreased with the addition of nanoparticles Al 2 O 3 , particularly in nanocomposites containing treated nanoparticles Al 2 O 3 with the crosslinker. This could be attributed to adequate adhesion between nanoparticles Al 2 O 3 and EPDM/SBR rubber blend, and it is clear that the lower the Q filler /Q gum , the greater the extent of interaction between nanoparticles Al 2 O 3 and EPDM/SBR rubber blend matrix. TESPT is a bi-functional silane coupling agent operating in two ways for the Al 2 O 3 -based EPDM/SBR system. In one way, TESPT interacted with the surface hydroxyl group of Al 2 O 3 to reduce the hydrophilic nature. TESPT, on the other hand, increased the possibility of sulfur crosslinking during the curing process of Al 2 O 3 -based EPDM/SBR compounds due to their sulfur-donating capacity. Thus, the two ways provided a clear explanation for the greater crosslink density of the EPDM/SBR sample in the presence of TESPT. Both maximum rheometric torque and tensile modulus are proportional to the crosslink density of rubber compounds. Figure 3 shows that the DC conductivity improves significantly after incorporating microparticles Al 2 O 3 into the EPDM/SBR rubber blend. As microparticle Al 2 O 3 loading increases, the interface regions between neighboring particles might overlap. Subsequently, a local conductive channel is formed inside the blend. However, the blend loaded with 8 phr nanoparticles Al 2 O 3 demonstrates higher DC conductivity than the other micro-counterparts. The contributed nanoparticles Al 2 O 3 metal purposes more charge carriers pathway into insulated rubber matrix, and therefore DC conductivity is eventually improved. Obviously, "the reference sample" filled with 50 phr CB demonstrates higher values ~ 3 × 10 -7 S/cm than all untreated/treated rubber vulcanizates.

DC Conductivity
MAH treatment enhanced the DC conductivity of both micro/nanoparticles Al 2 O 3 vulcanizates due to the enhancement of the interfaces between SBR and EPDM domains. This, in turn, makes it easier for the charge carriers to pass from one phase to the next. On the other hand, the vulcanizates' DC conductivity decreases after adding a crosslinker, ascribing to a decrease in carrier mobility [1]. To our knowledge, crosslinker can establish many traps between the interface of the Al 2 O 3 particles and the host rubber blend matrix. Then the movable charge carriers are trapped by the interfaces' traps, decreasing DC conductivity.

Permittivity
The permittivity ε' of EPDM/SBR (50/50) loaded with untreated /treated Al 2 O 3 particles at room temperature ~ 30 °C at three fixed frequencies, 0.1 k Hz, 1 kHz, and 10 kHz, are demonstrated in Fig. 4. It is clear that the permittivity ε' depends on micro/nano Al 2 O 3 content and increases with increasing their EPDM/SBR blend content. The ε' values are higher at lower frequencies (0.1 kHz) than those at higher frequencies. The increase in ε′ with increasing filler content and/or decreasing frequency of the applied electric field is a usual behavior in heterophase materials ascribed to Maxwell-Wagner Sillars (MWS)/ or interfacial polarization resulting from the existence of trapped charges within the system boundaries [27,33]. On the other hand, ε' values are enhanced after MAH addition; this is true for the vulcanizates containing micro/nanoparticles Al 2 O 3, respectively. MAH likely enhances the interfacial bonding between Al 2 O 3 and the rubber blend matrix. Moreover, the values of ε' were reduced after adding the crosslinker. Figure 5 and Fig. 6 show the variation of loss tangent tan δ and dielectric loss ε'' for EPDM/SBR (50/50) loaded with untreated /treated Al 2 O 3 particles at room temperature ~ 30 °C at three fixed frequencies: 0.1 k Hz, 1 kHz, and 10 kHz respectively. Generally, the values of loss tangent tan δ and dielectric loss ε'' for EPDM/SBR (50/50) loaded with untreated /treated Al 2 O 3 particles are lower than the reference sample loaded with CB. It can also be observed that the values of tan δ & ε'' increase as Al 2 O 3 loading increases or after MAH treatment. This increase in tan δ & ε'' with increasing Al 2 O 3 loading is associated with the interfacial polarization effects [34]. Interfacial polarization "Maxwell-Wagner-Sillars (MWS) effect" is detected in heterogeneous systems created from two or more phases. Al 2 O 3 particles in the EPDM/SBR rubber blend matrix are believed to increase the system's heterogeneity.
Otherwise, tan δ & ε'' of nanoparticles Al 2 O 3 composites are lower than microparticles Al 2 O 3 composites. This means nanoparticles Al 2 O 3 composites show more insulative behavior than the other micro counterparts. Taking 10 phr nanoparticles Al 2 O 3 composite samples as an example, its permittivity

Morphology of Rubber Blend Nanocomposites
SEM images of the fracture surfaces of composite rubbers reinforced with CB and Al 2 O 3 particles without and with surface treatment are shown in Fig. 7. Figures 7(c), (d), and (e) show that the size of untreated alumina agglomerates in Al 2 O 3 /EPDM/SBR rubber blend nanocomposites is much larger than that of treated alumina. The smaller aggregate size in the latter vulcanizates correlates well with the effect of interface modification in decreasing filler-filler interactions (caused by polar hydroxyl groups), increasing filler-polymer interactions, and thus achieving better dispersion of the treated alumina in the EPDM/SBR rubber blend matrix compared to that of the untreated alumina.
The sample containing carbon black was higher distribution than Al 2 O 3 due to good interaction and dispersion, as shown in Figs. 7(b), (c), and (d). Also, the nanoparticles of Al 2 O 3 were higher than the microparticles of Al 2 O 3, as shown in Figs. 7(g) and (h). In addition, the samples that used MAH as a compatibilizer were higher than those that used crosslinker, as shown in Figs. 7(e) and (f). The uniformly distributed fillers in the EPDM/SBR rubber blend matrix can resist the crack initiated and propagates, resulting in the composite's better tensile properties [35]. The uniform distribution of nanofillers enhances the properties of the nanocomposite [36,37]. The aggregated nanofillers develop a stress concentration point, resulting in a dropdown  [32]. The addition of the silane binding agent to the blend improves filler dispersion and decreases the filler-filler interface, resulting in the contact areas between rubber and fillers increase, which results in an improvement of filler-rubber interface interaction [14,30,38].

Conclusion
We prepared and compared Al 2 O 3 /EPDM/SBR with the current study's CB/EPDM/SBR rubber blend nanocomposites. Comparing composites with and without silane coupling agent on the mechanical properties, DC conductivity, dielectric properties, and EPDM/SBR rubber blend nanocomposites' swelling. In addition, 25 phr of microparticles Al 2 O 3 with TESPT resulted in a roughly 66% increase in tensile strength, from 1.57 MPa to 2.62 MPa, and a 108% increase in elongation at break (%), from 234% to 486.86%. The samples that used MAH as a compatibilizer had higher distribution and filler-rubber interaction than crosslinker. The Al 2 O 3 /EPDM/SBR nanocomposites' swelling with silane coupling agent showed better results than those without silane coupling agent. The blend loaded with nanoparticle Al 2 O 3 demonstrates higher DC conductivity than the other micro-counterparts. MAH treatment enhanced the DC conductivity more than the crosslinker of both micro/nano Al 2 O 3 vulcanizates due to the interfaces between SBR and EPDM domains. From the analysis, the presence of TESPT can increase the dispersion capability of Al 2 O 3 particles and their interfacial interaction with the EPDM/SBR matrix blend.