Abstract Acrylonitrile–butadiene rubber (NBR), a synthetic rubber having C≡N dipoles, was chosen as a polymer matrix with a higher dielectric constant than other non-polar rubber like silicone rubber or ethylene–propylene–diene monomer. Barium titanate (BaTiO3), as a ferroelectric material, with a high dielectric constant and low dielectric loss was selected as a main filler to further enhance the dielectric constant of NBR. An effective silane coupling agent (KH845-4), selected from five types of silane coupling agents with different characteristic functional groups, was used to modify the surface of BaTiO3 particles to enhance its interfacial adhesion to the matrix. Fourier transform infrared spectroscopy (FTIR) was used to verify the successful modification. The addition of BaTiO3 obviously enhanced the dielectric constants. In particular, an uncommon pattern of dielectric loss has been displayed and analyzed in this paper. Nevertheless, the reinforcing effect of mechanical strength of the NBR/ treated BaTiO3 composites is limited. On this basis, the addition of nanosilica (SiO2), replacing part of NBR, improved the mechanical strength. Confirmed by scanning electron microscopy (SEM), the SiO2 and treated BaTiO3 particles were dispersed well in the NBR matrix. The tensile strength was increased from 4.33 to 6.12 MPa when SiO2 accounted for 4%. Moreover, the curing characterizations, crosslinking density, resistivity, and oil resistance were evaluated. This composite material can be used in manufacturing electronic devices, which are subjected to oily environments for a long time.
Introduction Inorganic dielectric materials have been developed for several decades because of their high dielectric constant and low dielectric loss. They can be used for many applications such as base-plate materials, multi-layer ceramic capacitors (MLCCs), and resonators or filters in microwave communication. Nevertheless, these inorganic materials, particularly ceramic materials, are limited by their low breakdown strength and difficult processability [1, 2]. Therefore, many researchers have turned their attentions toward other materials including polymer materials [3, 4]. Compared with conventional ceramic dielectrics, polymer dielectrics have the main advantages such as light weight, excellent mechanical properties, chemical stability, and improved processability [5, 6]. Among the numerous polymer materials, elastomer materials have attracted more attention. Dielectric elastomers, with high actuated strain, can be designed for diverse applications like artificial muscle in bionic applications, valves, and actuators [7, 8]. Unfortunately, most polymer materials possess relatively low dielectric permittivity values despite some polar polymers like acrylonitrile–butadiene rubber (NBR) having permanent dipole moments. In electronic devices industry, greater functionality at smaller size is an essential requirement, which requires the dielectric materials for efficiency improvement and reliability . If the dielectric constant of a dielectric material is 4 times higher than that of the other, its size can be reduced to a half the size of the other [10, 11]. One of the basic methods to improve the dielectric
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constant is the organic/inorganic hybrid approach . Barium titanate (BaTiO3), as a ferroelectric material, with a high dielectric constant is usually used as a filler to add into dielectric polymers [12–14]. Salaeh et al.  reported a BaTiO3/epoxidized natural rubber (ENR-50) composite in which the dielectric constant increased with BaTiO3 loading level. Asimakopoulos et al.  added BaTiO3 nanoparticles into polyester resin in which the same conclusions could be drawn. However, the BaTiO3 particles may tend to aggregate and influence the overall properties of polymer composites, especially the mechanical strength, due to different polar surface tension between polymer matrix and inorganic filler. To solve this problem, two main techniques are effective. One is to use coupling agents to modify the surface of inorganic filler and the other is to add reinforcing fillers . In our previous work , ethylene–propylene–diene monomer (EPDM) was chosen as the matrix, with excellent resistance to heat and oxidation. The addition of BaTiO3 particles enhanced the dielectric constant noticeably. The EPDM composites can be manufactured in electronic devices applied in many particular conditions especially in high-temperature environment. Moreover, Bele et al.  selected silicone rubber (MQ) as a dielectric elastomer matrix to research energy-harvesting technology. Among this class of materials, silicone attracted their interest because of its high elasticity and weather resistance. The silicone elastomer can be used in outdoor environment. Neudys et al.  chose natural rubber (NR) to manufacture electronic devices due to its lowtemperature flexibility, high elasticity, fatigue resistance, tearing resistance, low heat buildup, and building tack. However, EPDM, MQ, and NR are non-polar polymers. They all have relatively low dielectric constant without ferroelectric filler and are unsuitable to employ in oily environment. In this study, NBR was selected as the matrix material. There was little research on NBR in dielectric application before. NBR has excellent oil resistance properties over a wide range of temperatures [21, 22]. As a dielectric polymer material, the C≡N dipole of NBR brings in significant orientation polarization . In other words, NBR has a relatively high dielectric constant compared with most non-polar polymers. Five types of silane coupling agents with different functional groups were chosen to modify the surface of BaTiO3 functional filler particles, and appropriately one was chosen in this study. On this basis, a small amount of nanosilica (SiO2) was added into this system for further mechanical reinforcement. Fourier transform infrared spectroscopy (FTIR) was used to characterize the modification of BaTiO3. The dispersion of SiO2 and BaTiO3 in the matrix was observed by scanning
electron microscopy (SEM). Curing characterizations determined the vulcanization conditions and crosslinking density characterized the degree of crosslinking. The mechanical properties and dielectric properties were the most important in this study. Moreover, the resistivity and oil resistance were evaluated.
Experimental Materials Acrylonitrile–butadiene rubber (CHK-26) with 26 wt% acrylonitrile content (AN) and the Mooney viscosity of 65–75 was supplied by Russia. Sulfur, zinc oxide, stearic acid, N-cyclohexyl-2-benzothiazole sulfonamide (accelerant), disulfide-benzothiazole (accelerant), and 2-mercapto benzimidazole (antioxidant) were commercially available. Barium titanate (particle size: D50 <1.78 μm, d = 6.02 g/cm3, crystalline phase: tetragonal) was purchased from SongBao Electronic Functional Materials Co., Ltd, Foshan, China. Vinyltriethoxysilane (SG-Si151), vinyltrimethoxysiloxane homopolymer (SG-Si6490), and bis-(γ-triethoxysilylpropyl)-tetrasulfide (KH845-4) silane coupling agents were supplied by Nanjing Shuguang Chemical Group Co., Ltd, Nanjing, China. 3-Aminopropyltriethoxysilane (KH550) and γ-methacryloxypropyltrimeth oxysilane (KH570) silane coupling agents were purchased from Sinopharm Chemical Reagent Co., Ltd, Shanghai, China. TS-530 nanosilica (d = 2.2–2.3 g/cm3), 1,1,1-trimethyl-N-(trimethylsilyl)-silanamine as surface modifier, was purchased from CABOT Co., Massachusetts, USA. ASTM 1# oil was supplied by Nanjing 7425 Rubber and Plastic Co., Ltd, Nanjing, China. Sample preparation Surface modification of BaTiO3 Five types of silane coupling agents (SG-Si151, KH550, KH570, SG-Si6490, and KH845-4) were used to modify the surface of BaTiO3 particles, respectively. Silane coupling agent (1.0 g) was first mixed with absolute ethyl alcohol (100 mL) for 15 min to make a homogeneous solution. Then, BaTiO3 (100 g) was gradually added into the solution with continuous stirring. A further stirring for 45 min after adding all BaTiO3 particles was conducted to prevent the BaTiO3 particle precipitation and to make BaTiO3 thoroughly in contact with the coupling agent molecules. The obtained mixture was dried for 6 h at 70 °C in a vacuum oven. In this procedure, only absolute ethyl alcohol was vaporized, and all 1 wt% silane coupling
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agents were physically or chemically adhered to the particle surface. Preparation of NBR compound and NBR/BaTiO3/SiO2 composites NBR compound included 100 phr NBR, 1.5 phr sulfur, 5 phr ZnO, 2 phr SA, 1 phr accelerant CZ, 1 phr accelerant DM, and 1 phr antioxidant MB. The volume fraction of untreated BaTiO3 and BaTiO3 treated by different silane coupling agents in preliminary experiments was 30 vol% and that of NBR compound was 70 vol%. In main experiments, SiO2 (1–5 vol%) that was added to reinforce the system reduced the corresponding volume fraction of NBR. The neat NBR compound was used as the control. Each NBR sample was mixed by a two-roll mixing mill (Shanghai Rubber Machinery Works, China). All NBR gums were vulcanized by hot pressing at 150 °C for 15 min, with a pressure of 10 MPa. The vulcanization conditions were according to the measurement of curing characteristics. Characterization Fourier transform infrared spectroscopy (FTIR) The chemical groups of BaTiO3 before and after being treated with different silane coupling agents were evaluated by a spectrometer (Nicolet Nexus 670, America). The spectra were recorded from 4000 to 400 cm−1, with a resolution of 2 cm−1. Curing characterization Curing characteristics of the NBR gums were determined at 150 °C for 20 min using an intelligent computer moving die rheometer (MDR 2000, Wuxi Liyuan Electronic and Chemical Equipment Co., Ltd, China). The optimum cure time (t90), scorch time (t10), minimum torque (ML), maximum torque (MH), and cure rate index (100/(t90 − t10)) were obtained from the curing curves. Mechanical properties The tensile and tear tests were carried out using an electromechanical universal testing machine (CMT 5254, Shenzhen SANS Testing Machine Co., Ltd, China), at a stable rate of 500 mm/min, according to ISO37 and ISO34, respectively. The Shore hardness of the specimens was tested with a rubber Shore A hardness degree tester (LXA, MingZhu Testing Machinery Co., Ltd, China) following ISO 868.
Scanning electron microscopy (SEM) The morphology of the fracture surface of samples and dispersion of BaTiO3 and SiO2 were observed by a JEOL SEM (5900, Japan). All the samples were fractured in liquid nitrogen, and the fracture surface was sputtered with a layer of gold before scanning. Crosslinking density Swelling test was carried out in toluene for 48 h at room temperature. After that, the samples were removed from the solvent while the surface toluene was quickly blotted off using tissue paper. The crosslinking density was calculated using Eq. (1):
ln (1 − Vr ) + Vr + χVr2 1 , Vr 3 Vs Vr − 2
where ν is the crosslinking density of the NBR composites (mol/cm3), Vr is the volume fraction of NBR after immersion in toluene, Vs is the molar volume of the swelling solvent (106.1 cm3/mol for toluene), and χ is the Flory–Huggins (rubber–toluene) interaction parameter (0.435 for the NBR–toluene system) . The volume fraction (Vr) of the vulcanizates was calculated using Eq. (2):
ms − mu ρr 1 =1+ · , Vr mu ρs
where ms is the weight of NBR rubber in swollen state after equilibrium swelling in toluene, mu is the weight of NBR rubber in air, ρs is the density of the swelling solvent (0.867 g/cm3 for toluene), and ρr is the density of NBR rubber. Dielectric properties Dielectric measurements were conducted according to IEC 60250 in the frequency range of 100–100 MHz. The equivalent shunt capacitance value (CP) and loss factor (D ) were measured by an Agilent precision impedance analyzer (4294A, America) at room temperature. The relative dielectric constant (εr) of the NBR composites was calculated using Eq. (3):
tCP , Sε0
where t is the thickness of the sample, S the area of the sample, and ε0 (8.845 PF/m) is the relative dielectric constant of air.
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The dielectric constant (ε′) and dielectric loss (ε′′) of the NBR gums were calculated according to the following equations:
ε′ = εr cos (arctgD)
ε′′ = εr sin (arctgD),
where the dielectric loss factor (tgδe) is equal to the loss factor (D). Volume and surface resistivity The volume and surface resistivity of the NBR composites were tested by a high-insulation resistance meter according to IEC 60093 which was purchased from Shanghai Precision and Scientific Instrument Co., Ltd, China. Oil and solvent resistance measurement Swelling tests in oil and solvent were conducted for the rubber composites according to ASTM D471-06. The weight (m0) and density (ρ0) of the initial samples were measured. Then the samples were immersed in ASTM 1# oil at room temperature for 120 h. For solvent swelling test, the samples were immersed in a mixture of toluene and isooctane solvent (1/3 and 3/1, v/v)  for 72 h. Each composite was selected in three samples with the same surface area of 20 mm × 20 mm × 1 mm for testing. The weight (m1) of each sample after different immersion periods was measured. The rate of volume change (Rv) was calculated using Eq. (6):
m1 −m0 ρL m0 ρ0
where ρL is the density of the oil or mixed solvent.
Results and discussion Choice of silane coupling agents FTIR analysis of BaTiO3 The silane coupling agents’ adherence to the surface of BaTiO3 was assessed by FTIR spectra with the untreated BaTiO3 being the control. Figure 1 shows the FTIR spectra of the untreated and treated BaTiO3. The important portion, between 2800 and 3000 cm−1, is magnified on the right for analysis. The peaks at 2925 and 2850 cm−1 are attributed to the asymmetric and symmetric C–H stretching in methylene groups, respectively. This result proves that the silane coupling agents were adhered to the surface
Fig. 1 FTIR spectra of untreated and treated BaTiO3
of BaTiO3 particles. Similarly, the new peak appearing at 1076 cm−1 which is due to the stretching vibration of Si–O can also confirm this conclusion. Additionally, the coupling agent KH570 has a characteristic peak at 1720 cm−1 (C=O stretching). The peak at 3446 cm−1, which has been detected in the spectrum of untreated BaTiO3, also proves the existence of –OH around the surface. Alkoxy groups can alcoholize with the hydroxyl groups on the particle surface and generate strong interactions between the particles and coupling agents. Effect of silane coupling agents on mechanical properties The mechanical properties of NBR filled with untreated and treated BaTiO3 are listed in Table 1. The addition of untreated BaTiO3 into NBR increased the hardness because the particles of BaTiO3 are rigid. The tensile strength, modulus at 100%, and tear strength decreased obviously due to the non-homogeneous dispersion of the inorganic particle in the polymer matrix. The elongation at break showed a slight enhancement because of the weak and physical adherence of untreated BaTiO3 to the rubber matrix. The improvement was observed when the BaTiO3 particles were treated by silane coupling agents. The increases in the hardness, tear strength, modulus at 100%, and tensile strength were obvious compared with the sample filled with untreated BaTiO3. Among the five types of silane coupling agents, KH845-4 is the most effective modifying agent. The addition of BaTiO3 treated with KH845-4 into NBR increased the tensile strength from 2.08 to 4.33 MPa and the tear strength from 12.92 to 21.81 kN/m compared with NBR filled with untreated BaTiO3. Although all the silane coupling agents can improve the dispersion of BaTiO3 in NBR, KH845-4 can also participate in the crosslinking reaction at the same time because it can provide sulfur element to this system. Nevertheless, the increase was still minor when NBR filled with KH845-4-treated BaTiO3 was
Iran Polym J Table 1 Mechanical properties of NBR with untreated and treated BaTiO3 Sample
compared with the control sample. According to the above analysis, some nanosilica particles were used as the reinforcing filler. Effect of silane coupling agents on dielectric properties Dielectric constant is a specific parameter of the degree of polarization under an applied alternating current or direct current field. The total polarizability of a dielectric is the sum of the contributions of each charge displacement produced in a polymer material. Generally, the dielectric constant of a composite material arises as a result of the polarization of the molecules. There are four main types of polarization: interfacial, orientation, atomic, and electronic polarization . Figure 2 represents the dielectric properties of NBR filled with untreated and treated BaTiO3 in a wide range of frequencies at room temperature. Firstly, all the samples filled with BaTiO3 show higher dielectric constant compared with the control sample. Although NBR is a polar polymer, its dielectric constant is smaller than those of most inorganic ferroelectric materials. The crystal of BaTiO3 has a tetragonal structure with ferroelectricity below the Curie temperature which is about 130 °C. Below the Curie temperature, BaTiO3 can occur the spontaneous polarization because of the asymmetric structure of its atomic arrangement in the crystal lattice . In the low-frequency region, the dielectric constant was relatively large and decreased sharply with increasing frequency. The permittivity of the control sample was over 13 at 100 Hz, and the permittivity of NBR filled with untreated BaTiO3 was over 24 at 100 Hz. This is mainly attributed to the orientation polarization owing to the presence of permanent dipoles in the NBR matrix (C≡N group) . In the highfrequency region, a different condition appeared unlike the low-frequency region, i.e., the dielectric constant was relatively small and decreased gently with the increase of frequency. This is due to the response time of orientation polarization. The orientation polarization requires more
time to reach the equilibrium static field compared with electronic and atomic polarizations. Therefore, as frequency increases, the decrease of dielectric permittivity is a result of the lag in orientation polarization . At the same time, electronic and atomic polarizations can reach the equilibrium static field as before, so there is hardly any change in what can be forecast at higher frequency beyond this experiment. Moreover, the spontaneous polarization of BaTiO3 is also a non-linear variation with frequency. Different silane coupling agents have had a slight effect on the dielectric constant because of just 1 wt% of BaTiO3 coupling agent present in the system. However, the use of coupling agents can improve the dispersion of BaTiO3 which, in turn, enhances the synergistic effect between BaTiO3 and NBR. Therefore, the dielectric constant of NBR filled with treated BaTiO3 has made a slightly better improvement compared to that of NBR filled with untreated BaTiO3. The dielectric loss factor of the control sample was higher than that of NBR/BaTiO3 composites because of the small dielectric loss factor of BaTiO3 and less rubber component present in these samples compared to the control sample. In the low-frequency region (around 100 Hz), a large dielectric loss was observed mainly due to the lag in interfacial polarization which resulted from migration and entrapment of charge carriers at the interfacial area. The response time of interfacial polarization is very long compared with those of other three types of polarization. Around this frequency region, the interfacial polarization was hardly reflected in permittivity because the response time exceeded the given time in this study. The most part of energy due to lag in interfacial polarization was translated into the thermal energy emitted into air, which was observed as the dielectric loss. An obvious loss peak in the high-frequency region (around 1 MHz) was also observed. This peak was attributed to the lag in the orientation polarization and to the chain segmental motion of the rubber. Prior to this frequency region, the orientation polarization can keep up with the change in electric field completely. The dielectric loss
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Fig. 2 Dielectric properties of NBR with untreated and treated BaTiO3
caused by orientation polarization is relatively small. In this frequency region, the orientation polarization can hardly keep up with the change in electric field. The dielectric loss caused by orientation polarization is also relatively small. Between these two terminal conditions, a loss peak may appear. Hence, prior to 100 Hz, a loss peak can be predicted because of the same reason as for interfacial polarization. However, dielectric loss of the control sample was lower than that of the other samples due to their different dielectric constant values. The synergistic effect of NBR and BaTiO3 produced a similar pattern in dielectric properties of NBR and NBR/BaTiO3 composites. Different silane coupling agents had a slight effect on the dielectric properties. Hence, according to the results of the effects on the mechanical and dielectric properties, BaTiO3 treated with KH845-4 was chosen as the major filler to carry out the following work.
Effect of BaTiO3 and SiO2 on cure properties of NBR The cure characteristics of NBR filled with BaTiO3 and SiO2 are listed in Table 2. The addition of BaTiO3 into NBR increased its minimum torque (ML), maximum torque (MH), and the difference between them. The increase in the maximum torque can be accounted for by two possible factors: (1) a small increase in physical crosslinking or reinforcement due to the addition of BaTiO3; (2) the fillers may form agglomerates which ultimately increase the stiffness of the composites. It is well known that BaTiO3 is a kind of rigid filler and its addition into the rubber enhances the MH value which is similar to the hardness of material. The ML, MH, and the difference between maximum and minimum torque of NBR filled with BaTiO3, treated by KH8454, were higher than those of NBR filled with untreated
ML minimum torque, MH maximum torque, t90 optimum cure time, t10 scorch time; cure rate index = 100/ (t90 − t10)
BaTiO3. As for KH845-4, its polysulfur bonds can participate in crosslinking. Rubber with a higher degree of crosslinking exhibits lower flexibility and elasticity. Therefore, the MH value is increased further. The interesting phenomenon was that the ML and MH values were reduced due to the addition of 1 vol % silica. This is because, despite the fact that the silica fillers were modified by coupling agents, a few acidic silanol groups on the surface of the silica fillers remained unchanged. In the composites with the KH845-4 silane coupling agent, a large number of acidic silanol groups entered reactions depending on the concentration of silane coupling agent. Therefore, the concentration of silanol group was obviously reduced. However, the ML and MH values gradually increased with further addition of silica because the amount of silane coupling agent remained constant with the increase of silica. At the same time, adding silica also led to an increase in maximum torque because of the two above-mentioned reasons. Moreover, the presence of acidic silanol groups on the surface of silica fillers always retarded the vulcanization reaction because of acidity . Hence, the cure rate index became smaller with increasing the silica content. Analysis of particle dispersion in BaTiO3 and SiO2/ NBR composites The SEM micrographs of the fracture surface for the NBR composites are shown in Fig. 3. Due to the strong hydrogen bonds formed by hydroxyl groups on the BaTiO3 particle surface, the untreated BaTiO3 showed a tendency to agglomerate. Many irregular holes were left due to the aggregation of BaTiO3 particles. Such defect can be reflected in some other properties of the composites. After surface treatment by KH845-4, the agglomeration of BaTiO3 particles could be reduced and even eliminated. The reason is that the presence of KH845-4 on the surface of BaTiO3 particles changes the surface energy of BaTiO3
and the functional groups of KH845-4 can generate chemical bonds with the NBR matrix. The problem is BaTiO3 itself which has comparatively large particles with an inhomogeneous distribution. However, there is no doubt that saline coupling agent can improve the filler dispersion and reduce the agglomeration of particles . Similar results are observed in Fig. 3d–f. Moreover, at the top-right corner of each image, some different scenes from six close-up images are observed. Figure 3a is very neat. In Fig. 3b, c, except the larger BaTiO3 particles, there is no other particle. Nevertheless, excluding the larger BaTiO3 particles, some small particles are also observed in Fig. 3d–f. On the other hand, more small particles are found in Fig. 3e, f compared with Fig. 3d. According to the formulation and the results observed in Fig. 3, these small particles correspond to SiO2 particles. The other powerful evidence is that the size of silica particles is the nanoscale range in this system. Meanwhile, these nanosilica particles have displayed good dispersion because they are hydrophobic. Effect of BaTiO3 and SiO2 on crosslinking density of NBR composites In swelling test, the crosslinking densities of the vulcanized NBR composites were calculated and are summarized in Table 3. These results follow the order: NBR filled with KH845-4-treated BaTiO3 and SiO2 > NBR filled with KH845-4-treated BaTiO3 > NBR control > NBR filled with untreated BaTiO3. The addition of untreated BaTiO3 has broken the crosslinked structure of NBR because of the agglomeration of BaTiO3 particles. Nevertheless, the use of KH845-4 improved the dispersion of BaTiO3 particles and provided the crosslinking points due to the presence of polysulfur in the molecular structure of KH845-4. The addition of SiO2 has led to the generation of extra physical crosslinking points. Furthermore, the crosslinking densities increased with increasing the silica content.
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Fig. 3 SEM micrographs of NBR vulcanizates: a NBR control, b NBR/BaTiO3 (untreated), c NBR/BaTiO3 (KH548-4), d NBR/BaTiO3 (KH548-4)/1% SiO2, e NBR/BaTiO3 (KH548-4)/3% SiO2, and f NBR/BaTiO3 (KH548-4)/5% SiO2 Table 3 Crosslinking density of NBR/BaTiO3 (KH845-4-treated)/ SiO2 composites Sample
Effect of SiO2 on mechanical properties of NBR composites The mechanical properties of the NBR composites are shown in Fig. 4. The addition of silica can further increase the tensile strength, 100% modulus, and tear strength to some degree on the basis of NBR/BaTiO3 (treated with KH845-4) composite. Figure 5 illustrates the crosslinking and dispersion models of BaTiO3 and nanosilica particles in the NBR matrix. Figure 5a shows the crosslinking model of neat NBR. Sulfur can react with the active hydrogen atom in carbon atom bordered with the C=C bond, belonging to the butadiene components in NBR. The reaction can form polysulfur
bonds between molecular chains. Figure 5b shows the effect of the addition of untreated BaTiO3. The addition of untreated BaTiO3 has decreased the tear strength and tensile strength of the NBR composites because of agglomerated BaTiO3 particles. A large amount of agglomeration has destroyed the structure of continuous phase, generating many defects in integral structure combining with the result of crosslinking density. However, it is observed that the weak and physical adherence of untreated BaTiO3 particles to the NBR matrix could increase the elongation at break to some degree. Figure 5c displays the effect of surface modification on BaTiO3. The surface modification of BaTiO3 by KH845-4 brought about an obvious increase in tensile strength, elongation at break, 100% modulus, and tear strength compared with the untreated sample because the crosslinking density of NBR filled with KH845-4-treated BaTiO3 is higher than that of NBR filled with untreated BaTiO3, as shown in Table 3. This result is attributed to the polysulfur bonds and the functional groups of KH845-4 which can participate in crosslinking during the vulcanization course. Simultaneously, saline coupling agent may improve the filler dispersion and reduce the agglomeration of particles, resulting in reduced defects in the structure of composites. However, compared to the control sample, the improvement in the mechanical properties is trivial. As one of the most common reinforcing fillers, silica is usually used to reinforce silicone rubber. For other rubbers, silica needs to be modified first. In this study, TS530,
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Fig. 4 Mechanical properties of NBR/BaTiO3 (KH845-4-treated)/SiO2 composites: a tensile strength and elongation at break; b modulus at 100% and tear strength; and c permanent set and hardness
a commercial silica, was chosen to modify by 1,1,1-trimethyl-N-(trimethylsilyl)-silanamine. It was observed that the tensile strength, 100% modulus, and tear strength were improved by adding the treated TS530 silica. Moreover, the mechanical properties were further increased with increasing the silica content. The tensile strength and tear strength of the sample filled with 5 vol% silica increased from 4.33 to 6.02 MPa compared with those of the NBR/ BaTiO3 (treated by KH845-4) composites, which increased from 21.81 to 27.93 kN/m. The main reason was that the active hydroxyl groups on the surface of SiO2 particles could act as strong adsorption sites. These nanoparticles could act as physical crosslinking points located in the narrow gaps between two adjacent macromolecular chains or in the crispation of a simple chain, as shown in Fig. 5d. The effect can form the space net structure between macromolecular chains and SiO2. However, a decrease in the elongation at break and a small increase in the permanent set were observed because of the rigid silica particles which could
make the NBR gums lose their flexibility. This was the reason why we did not further increase the silica content. The results of hardness can be associated with the MH value of cure characteristics. Effect of SiO2 on dielectric properties of NBR gums Figure 6 shows the curves of dielectric properties for the NBR/BaTiO3 (treated by KH845-4)/SiO2 composites. In general, a small amount of nanosilica (1–5 vol%) shows a slight effect on the dielectric properties. In the low-frequency region, as shown in Fig. 6a, these six curves follow a certain order as a whole in the low-frequency region: the dielectric constant becomes smaller with increasing the silica content. This is because silica is used to substitute NBR partially as silica is a type of non-polar filler and it is not an inorganic ferroelectric material with dielectric constant smaller than that of NBR in the low-frequency region. The lower content of NBR matrix can give rise to a smaller
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Fig. 5 Crosslinking and dispersion models of NBR/BaTiO3 (KH845-4-treated)/SiO2 composites: a NBR control, b NBR with untreated BaTiO3, c NBR with KH845-4-treated BaTiO3, d NBR with KH845-4-treated BaTiO3 and SiO2
degree of orientation polarization. In the high-frequency region, the small addition of SiO2 has a slight effect on the dielectric constant because the dielectric constant of pure silica is slightly larger than that of NBR and the orientation polarization belonging to NBR has hardly any effect on the dielectric constant of these composites. The dielectric loss and dielectric loss factor showed a similar pattern such as those illustrated in Fig. 2. Moreover, the dielectric loss and dielectric loss factor decreased by increasing the volume fraction of SiO2. It was also caused by the decrease of NBR component in the system, in which the lag of orientation polarization was also decreased.
Effect of BaTiO3 and SiO2 on volume and surface resistivity of NBR gums Table 4 shows the volume and surface resistivity of the NBR composites. Resistivity indicates the resistance of a material to passing electric current. Volume resistivity is mainly used to characterize the insulation performance of materials. The purpose of this study was to obtain an NBR dielectric elastomer composite with a high dielectric constant and a low dielectric loss factor. Therefore, the dielectric elastomer should be included in a scope of insulator. Surface resistivity is often used to characterize the
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Fig. 6 Dielectric properties of NBR/BaTiO3 (KH845-4-treated)/SiO2 composites Table 4 Volume and surface resistivity of NBR/BaTiO3 (KH845-4-treated)/SiO2 composites
electrical property of the surface of different materials. It is easily influenced by various environmental factors. Volume resistivity is more important than surface resistivity; therefore, the surface resistivity has been measured and used in this study. The volume resistivity and surface resistivity values of the control sample were measured to be of 6.54 × 108 Ω m and 5.14 × 1012 Ω, respectively. These
values were smaller than those associated with general nonpolar polymer materials due to its polar functional group (–CN). The incorporation of 30 vol % untreated BaTiO3 did not generate any obvious effect on the resistivity compared with the control sample, but KH845-4-treated BaTiO3 had an effect on the resistivity that turned into a half of the control sample, either volume or surface resistivity. This
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Table 5 Volume change (%) of NBR immersed in oil and solvent for different immersion periods Sample
is correlated with the amount of NBR matrix adhered to the particle surface. Higher adherence rate revealed the formation of more electric current passage. In the meantime, the small addition of silica particles has had hardly any effect on the volume and surface resistivity of the NBR composites. Effect of BaTiO3 and SiO2 on oil and solvent resistance of NBR gums The volume change of the NBR gums immersed in ASTM 1# oil and mixed solvent for different immersion periods is summarized in Table 5. The excellent oil resistance was proved by the experimental data. Even though being immersed in the oil for 120 h, there was only a 1.8 percent increase in volume for the control sample due to the existence of C≡N dipoles. Moreover, other samples have displayed a smaller change compared to the control sample because of less NBR content with the same volume. From the results of solvent resistance test, similar results were observed related to the crosslinking density data. There was a considerable increase in the volume change for the gums immersed only for 4 h in partial polar solvent
(Vtoluene:Viso-octane = 3:1) because the solubility parameters of NBR and the mixed solvent were close to each other. While all the samples were immersed for 72 h, a slight increase in the volume change was measured when compared with the sample immersed for 4 h. For the relatively weak polar solvent (Vtoluene:Viso-octane = 1:3), the increase in the volume change of all the samples was gentle for 72 h and the volume change was smaller than that of the samples immersed in a partial polar solvent. In general, the volume change of the control sample and the NBR/untreated BaTiO3 sample was larger than those of the other samples because of lower crosslinking density of NBR/BaTiO3 (untreated) and the more NBR component present in the same volume of the control sample. In combination, the surface modification of the BaTiO3 particles could improve the oil and solvent resistance of the composites.
Conclusion Incorporation of 30 vol % untreated BaTiO3 into NBR obviously enhanced the dielectric constant but caused a significant drop in the mechanical properties. To solve the shortage, different silane coupling agents were used to modify the BaTiO3 particles. It was found that just 1% coupling agent of KH845-4 (by weight of BaTiO3) increased the tensile and tear strength from 2.08 to 4.33 MPa and 12.92–21.81 kN/m, respectively, compared with the sample filled with untreated BaTiO3, because surface modification of the inorganic particles improved the filler dispersion and reduce the agglomeration of particles in the NBR matrix. At the same time, the polysulfur present in the molecular structure of KH845-4 participated in the crosslinking reaction to generate chemical bonds between the BaTiO3 particles and NBR matrix, enhancing the crosslinking density of NBR composites. The small addition of nanosilica further improved the mechanical properties, because these particles could act as physical crosslinking points. Both the silane coupling agents and nanosilica have had a slight effect on the dielectric properties. In the low-frequency region, the dielectric constant was relatively large and decreased sharply with increasing frequency. In the highfrequency region, the dielectric permittivity was relatively small and decreased gently with increasing frequency. The 30 vol % of BaTiO3 loading resulted in a more than double change in the permittivity in a wide range of frequencies. It is well known that NBR has excellent oil resistance which was confirmed by the data from oil resistance test. Hence, these NBR composites with high dielectric constant can be applied to manufacture many types of electronic devices to function in oily environments without any problem for a long time.
Iran Polym J Acknowledgements This work was supported by the Innovation Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD).
References 1. Parizi SS, Conley G, Costanzo T, Howell B, Mellinger A, Caruntu G (2015) Fabrication of barium titanate/acrylonitrile-butadiene styrene/poly(methyl methacrylate) nanocomposite films for hybrid ferroelectric capacitors. RSC Adv 5:76356–76362 2. Abe K, Nagao D, Watanabe A, Konno M (2013) Fabrication of highly refractive barium-titanate-incorporated polyimide nanocomposite films with high permittivity and thermal stability. Polym Int 62:141–145 3. Chen T, Qiu J, Zhu K, Li J (2016) Electro-mechanical performance of polyurethane dielectric elastomer flexible micro-actuator composite modified with titanium dioxide-graphene hybrid fillers. Mater Des 90:1069–1076 4. Bur AJ (1985) Dielectric-properties of polymers at microwavefrequencies: a review. Polymer 26:963–977 5. Zhi X, Mao Y, Yu Z (2015) Gamma-aminopropyl triethoxysilane functionalized graphene oxide for composites with high dielectric constant and low dielectric loss. Compos A: Appl S 76:194–202 6. Luo H, Wu Z, Chen C, Ma C, Zhou K, Zhang D (2016) Methoxypolyethylene glycol functionalized carbon nanotube composites with high permittivity and low dielectric loss. Compos A: Appl S 86:57–65 7. Jiang L, Betts A, Kennedy D, Jerrams S (2015) The fabrication of dielectric elastomers from silicone rubber and barium titanate: employing equi-biaxial pre-stretch to achieve large deformations. J Mater Sci 50:7930–7938 8. Sheng J, Chen H, Li B, Wang Y (2013) Influence of the temperature and deformation-dependent dielectric constant on the stability of dielectric elastomers. J Appl Polym Sci 128:2402–2407 9. Poh CL, Mariatti M, Noor AF, Sidek O, Chuah TP, Chow SC (2016) Dielectric properties of surface treated multi-walled carbon nanotube/epoxy thin film composites. Compos B Eng 85:50–58 10. Wersing W (1996) Microwave ceramics for resonators and filters. Curr Opin Solid St M 1:715–731 11. Cava RJ (2001) Dielectric materials for applications in microwave communications. J Mater Chem 11:54–62 12. Sharma P, Kumar P, Kundu RS, Juneja JK, Ahlawat N, Punia R (2015) Structural and dielectric properties of substituted barium titanate ceramics for capacitor applications. Ceram Int 41:13425–13432 13. Sharma S, Shamim K, Ranjan A, Rai R, Kumari P, Sinha S (2015) Impedance and modulus spectroscopy characterization of lead free barium titanate ferroelectric ceramics. Ceram Int 41:7713–7722 14. Luo H, Zhang D, Jiang C, Yuan X, Chen C, Zhou K (2015) Improved dielectric properties and energy storage density of poly(vinylidene fluoride-co-hexafluoropropylene) nanocomposite
with hydantoin epoxy resin coated BaTiO3. ACS Appl Mater Inter 7:8061–8069 15. Salaeh S, Boiteux G, Cassagnau P, Nakason C (2015) Flexible ceramic-polymer composites of barium titanate and epoxidized natural rubber. Int J Appl Ceram Technol 12:106–115 16. Asimakopoulos IA, Psarras GC, Zoumpoulakis L (2014) Barium titanate/polyester resin nanocomposites: development, structureproperties relationship and energy storage capability. Express Polym Lett 8:692–707 17. Tomovska R, Daniloska V, Asua JM (2013) Surface modification of TiO2 nanoparticles via photocataliticaly induced reaction: influence of functionality of silane coupling agent. Appl Surf Sci 264:670–673 18. Su J, Zhang J (2015) Remarkable enhancement of mechanical and dielectric properties of flexible ethylene propylene diene monomer (EPDM)/barium titanate (BaTiO3) dielectric elastomer by chemical modification of particles. RSC Adv 5:78448–78456 19. Bele A, Stiubianu G, Varganici C, Ignat M, Cazacu M (2015) Silicone dielectric elastomers based on radical crosslinked high molecular weight polydimethylsiloxane co-filled with silica and barium titanate. J Mater Sci 50:6822–6832 20. González N, Custal MA, Lalaouna S, Riba JR, Armelin E (2016) Improvement of dielectric properties of natural rubber by adding perovskite nanoparticles. Eur Polym J 75:210–222 21. Ahmad HS, Ismail H, Azura AR (2015) Comparison properties of natural rubber/virgin acrylonitrile-butadiene rubber and natural rubber/recycled acrylonitrile-butadiene rubber blends. Iran Polym J 24:185–195 22. Mirabedini AS, Karrabi M, Ghasemi I (2013) Viscoelastic behavior of NBR/phenolic compounds. Iran Polym J 22:25–32 23. Jiang H, Ni Q, Wang H, Liu J (2012) Fabrication and characterization of NBR/MWCNT composites by latex technology. Polym Composite 33:1586–1592 24. Ge X, Li MC, Cho UR (2015) Novel one-step synthesis of acrylonitrile butadiene rubber/bentonite nanocomposites with (3-Mercaptopropyl) trimethoxysilane as a compatilizer. Polym Composite 36:1693–1702 25. Rajasekar R, Pal K, Heinrich G, Das A, Das CK (2009) Development of nitrile butadiene rubber-nanoclay composites with epoxidized natural rubber as compatibilizer. Mater Des 30:3839–3845 26. Rajesh C, Manoj KC, Unnikrishnan G, Purushothaman E (2013) Dielectric properties of short nylon-6 fiber-reinforced NBR composites. Adv Polym Technol 321:E90–E102 27. Haertling GH (1999) Ferroelectric ceramics: history and technology. J Am Ceram Soc 82:797–818 28. Serra-Gomez R, Gonzalez-Gaitano G, Gonzalez-Benito J (2012) Composites based on EVA and barium titanate submicrometric particles: preparation by high-energy ball milling and characterization. Polym Composite 33:1549–1556 29. Dasgupta M, Das Gupta S, Mukhopadhyay R, Bandyopadhyay A (2014) A new silica-rich material from waste fly ash—generation, characterisation and study of its effectiveness as a filler for rubber compounds. Polym Polym Compos 22:569–580 30. Ismail H, Kheong OW (2008) The effect of bis-(3-triethoxysilylpropyl) tetrasulfide, Si69 on properties of recycled poly (vinyl chloride)/acrylonitrile-butadiene rubber/fly ash (PVCr/NBR/FA) composites. J Reinf Plast Comp 27:1649–1661