Enhancement in electrical and thermal performance of high-temperature vulcanized silicone rubber composites for outdoor insulating applications

High-temperature vulcanized silicone rubber composites are highly desirable as outdoor insulating materials due to their immense thermal and electrical performance. The aim of this work is to study the role of co-combined fillers (modified fumed silica [MFS], titanium dioxide [TiO₂], with graphene [G]) on electrical and thermal properties of silicone rubber (S) composites. The dielectric response of S/MFS_10 phr and S/TiO₂_20 composites tailored with 2 phr G was characterized by broadband dielectric spectroscopy. The hybrid filler/composites were found to show higher thermal stability when 2 phr G was added. In addition, a low quantity of G filler was found to slightly increase the AC dielectric breakdown strength of the S/MFS_10 and S/TiO₂_20, where an improvement of 3 and 5% was found, respectively. Several steps were observed in the thermal decomposition of the S rubber composites by thermogravimetric analysis-Fourier-transform infrared spectroscopy. Our findings revealed great potentials for fabricating hybrid-filler/silicone rubber composites with enhanced electrical and thermal properties for outdoor insulating applications.     

Influences of dielectric and conductive fillers on dielectric and mechanical properties of solid silicone rubber composites

Dielectric elastomers are materials being used for electromechanical transduction applications. Their electromechanical response depends on permittivity, Young’s modulus and electric breakdown strength. A factor that limits its application is high operating voltages that can be reduced through improvement in permittivity. One of the methods is by incorporating high permittivity fillers into polymer matrix to obtain dielectric–dielectric composites (DDC).These composites show high permittivity at the cost of reduced flexibility. An alternative solution is development of composites by incorporating organic or inorganic conductive fillers into polymer matrix. These composites show high permittivity with high dielectric loss and low breakdown strength. To overcome both the above limitations both dielectric and conductive fillers are incorporated into dielectric polymer matrix to obtain conductor–dielectric composites (CDC). In this study, high temperature vulcanized solid silicone rubber as matrix has been used to prepare DDC composites with barium titanate (BT) filler and CDC composites with both BT as dielectric and ketjenblack as conductive fillers, using Taguchi design of experiments. The effect of factors such as amount of fillers and curing agent, mixing time in roll mill and curing temperature on the dielectric and mechanical properties are reported. Lichtenecker model predicts the permittivity of the DDC composite more accurately. For the CDC composites permittivity increased by 390%, effective resistivity decreased by 80%, Young’s modulus increased by 368% and Shore A hardness increased by 90% as compared to those of reference matrix. Important interaction effects are observed among both the fillers that are uniformly dispersed without any aggregation.

     

Effect of different types of filler and filler loadings on the properties of carboxylated acrylonitrile–butadiene rubber latex films

The effects of different types of fillers and filler loadings on the properties of carboxylated nitrile rubber (XNBR) latex were identified. Silica, mica, carbon black (CB; N330), and calcium carbonate (CaCO₃) were used as fillers with filler loadings of 10, 15, and 20 parts per hundred rubber. Furnace ashing and Fourier transform infrared analysis proved that interaction existed between the fillers and XNBR latex films. The morphology of the filled XNBR films was significantly different for different types of fillers. Mica and CaCO₃ fillers showed uneven distribution within the XNBR film, whereas other fillers, such as silica and CB, showed homogeneous distribution within the films. In the observation, silica and mica fillers also illustrated some degree of agglomeration. The mechanical properties (e.g., tensile and tear strengths) showed different trends with different types of fillers used. For silica and mica fillers, the mechanical properties increased with filler loadings up to a certain loading, and decreased with higher filler loadings. For CB filler, the mechanical properties increased gradually with increasing filler loadings. CaCO₃ fillers did not increase the mechanical properties. The crosslinking density of the XNBR films increased when they were incorporated with fillers because of the presence of elastomer–filler and filler–filler interactions. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2010     

Composition dependence of dielectric properties,elastic modulus,and electroactivity in (carbon black‐BaTiO3)/silicone rubber nanocomposites

The dielectric properties, elastic modulus, and electromechanical responses of dielectric elastomers (DEs) consisting of silicone rubber and carbon black (CB) incorporated with BaTiO₃ (BT) were studied. When compared with single filler/rubber composites, the resulting three‐component nanocomposites yielded very abnormal phenomena. They might be attributed to the interactions between the two kinds of fillers. The increase in concentration of CB (BT) would play a destructive role to the network structure formed by BT (CB) particles. The maximum electromechanical strain of the nanocomposites achieved at mass fraction m_CB = 0.03 and m_BT = 0.06. The resultant electromechanical strain would be attributed to the large dielectric permittivity in the three‐component nanocomposites, in which the BT particles themselves have a high dielectric permittivity and the electrical networks of CB particles have a contribution on the increase in dielectric permittivity of the three‐component nanocomposites. © 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013     

Low dielectric loss LNT/PEEK composites with high mechanical strength and dielectric thermal stability

(3-Aminopropyl)triethoxysilane treated La_(2−x)/3Na_0.06TiO₃ (x = 0.06) (LNT) microparticles filled polyetheretherketone (PEEK) composites were prepared using hot pressing process. The effects of variation of LNT ceramic filling fraction on dielectric properties, water absorption, thermal stability and mechanical strength were investigated. All composites demonstrate low water absorption (less than 0.4%) when the ceramic filling fraction is lower than 0.6V_f. The obtained composites exhibited dielectric permittivities varying from ~4 to ~22 as the ceramic fillers increased from 0.1 to 0.8V_f and low losses (~10⁻⁴ @1 MHz, 3~5 × 10⁻³ at the frequencies of microwave (10 GHz) and millimeter wave (29-50 GHz), respectively). The mechanical strength, dimensional and dielectric thermal stability of the composite are remarkably improved by the addition of LNT ceramic fillers. A composite with near zero temperature coefficients of dielectric permittivity or resonant frequency and flexural strength of ~140 MPa could be obtained. The out-of-plane coefficient of thermal expansion (CTE) could be reduced to ~20 ppm/°C as the ceramic filler loading reached 0.7V_f.     

Post Curing as an Effective Means of Ensuring the Long-term Reliability of PDMS Thin Films for Dielectric Elastomer Applications

Post curing can be used to facilitate volatile removal and thus produce polydimethylsiloxane (PDMS) films with stable elastic and electrical properties over time. In this study, the effect of post curing was investigated for commercial silicone elastomer thin films as a means of improving long-term elastomer film reliability. The Young’s moduli and electrical breakdown strengths of commercial (silica-reinforced) PDMS elastomer films, with and without additional 35 parts per hundred rubber titanium dioxide (TiO₂), were investigated after high-temperature (200°C) post curing for various time spans. The elastomers were found to contain less than 2% of volatiles (significantly higher for TiO₂-filled samples), but nevertheless a strong effect from post curing was observed. The young’s moduli as well as the strain-dependent behavior were found to change significantly upon post curing treatment, where Young’s moduli at 5% strain increase with post curing. Furthermore, the determined dielectric breakdown parameters from Weibull analyses showed that greater electrical stability and reliability could be achieved by post curing the PDMS films before usage, and this method therefore paves a way toward more reliable dielectric elastomers.     

Improvement of actuation performance of dielectric elastomers by barium titanate and carbon black fillers

Dielectric elastomers are promising materials for actuators resembling human muscle. Among elastomers, acrylic rubbers (ACM) have shown good actuation performance but its use is limited by the high operating voltages required. The present work demonstrates that simultaneous incorporation of nanostructured carbon black and dielectric fillers offers an increase in a dielectric permittivity and a suitable modulus of the elastomers matrix, enabling an improved electro‐mechanical actuation performance at low voltages. By the use of reinforcing carbon black and barium titanate in an acrylic elastomer matrix a sixfold increase in the dielectric permittivity was realized. A fine tuning of the actuation stress and, consequently, actuation strain can be done by a judicial selection of the different filler concentrations in the soft rubber matrix. Finally, a synergistic effect of the fillers was observed in the improved actuation performance of the developed materials. This work may pave the way to design dielectric elastomers for actuator fabrication. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016 , 133, 44116.     

填料对双组分缩合型室温硫化硅橡胶性能的影响

考察了多种补强填料和半补强(或增量)填料如硅藻土、碳酸钙和氧化铝等对室温硫化(RTV)硅橡胶的机械性能和介电性能的影响。结果表明,采用经六甲基二硅氮烷处理的气相法白炭黑或沉淀法白炭黑作填料时,RTV硅橡胶多方面的性能都有所提高;采用补强填料和半补强填料并用的RTV硅橡胶的性能优于使用单一补强填料的RTV硅橡胶;随着填料用量的增加,RTV硅橡胶的机械性能、相对介电常数和介质损耗因数都有一定程度的提高,但体积电阻率有所降低。     

Fly ash particles and precipitated silica as fillers in rubbers. I. Untreated fillers in natural rubber and styrene–butadiene rubber compounds

In this study, we investigated the effects of untreated precipitated silica (PSi) and fly ash silica (FASi) as fillers on the properties of natural rubber (NR) and styrene–butadiene rubber (SBR) compounds. The cure characteristics and the final properties of the NR and SBR compounds were considered separately and comparatively with regard to the effect of the loading of the fillers, which ranged from 0 to 80 phr. In the NR system, the cure time and minimum and maximum torques of the NR compounds progressively increased at PSi loadings of 30–75 phr. A relatively low cure time and low viscosity of the NR compounds were achieved throughout the FASi loadings used. The vulcanizate properties of the FASi‐filled vulcanizates appeared to be very similar to those of the PSi‐filled vulcanizates at silica contents of 0–30 phr. Above these concentrations, the properties of the PSi‐filled vulcanizates improved, whereas those of the FASi‐filled compounds remained the same. In the SBR system, the changing trends of all of the properties of the filled SBR vulcanizates were very similar to those of the filled NR vulcanizates, except for the tensile and tear strengths. For a given rubber matrix and silica content, the discrepancies in the results between PSi and FASi were associated with filler–filler interactions, filler particle size, and the amount of nonrubber in the vulcanizates. With the effect of the FASi particles on the mechanical properties of the NR and SBR vulcanizates considered, we recommend fly ash particles as a filler in NR at silica concentrations of 0–30 phr but not in SBR systems, except when improvement in the tensile and tear properties is required. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 93: 2119–2130, 2004     

Enhanced dielectric constant and breakdown strength in dielectric composites using TiO2@HfO2 nanowires with gradient dielectric constant

To investigate the influence of modification of ceramic fillers on the dielectric properties of polymer-based composites, TiO₂ and core-shell structured TiO₂@HfO₂ nanowires were synthesized, and investigated in this study. TiO₂ nanowires/polyvinylidene fluoride (PVDF) and TiO₂@HfO₂ nanowires/PVDF nanocomposites were prepared using the solution casting method. The experimental results showed that the TiO₂@HfO₂ nanowires/PVDF composites had improved dielectric properties compared with that of the TiO₂ nanowires/PVDF composites. Owing to the enhanced interfacial polarisation by the multilevel interface, the composites with 10 wt % TiO₂@HfO₂ nanowires achieved the highest permittivity of 12.56 at 1 kHz, which was enhanced by ～72% compared to the PVDF matrix. The electric field was evenly distributed by building the fillers with a gradient dielectric constant. The characteristic breakdown strength of the composite with 5 wt % TiO₂@HfO₂ reached 377.76 kV/mm, compared with that of 334.37 kV/mm for the composite with 5 wt % TiO₂ nanowires. This study initiated a novel strategy for preparing dielectrics with high dielectric constant and improved breakdown strength.     

Dielectric relaxation of conductive carbon black reinforced chlorosulfonated polyethylene vulcanizates

The frequency dependent dielectric relaxation behavior of conductive carbon black reinforced chlorosulfonated polyethylene (CSM) vulcanizates has been studied for different filler loadings in the frequency range of 10²–10⁶ Hz over a wide range of temperatures (30–120°C). The effects of filler loadings on the dielectric permittivity (ε′), dielectric loss tangent (tan δ), impedance, and electrical conductivity were studied. The variation of the dielectric permittivity with the filler loadings was explained on the basis of interfacial polarization of the filler in the polymer matrix. The frequency dependence of ac conductivity has been investigated using percolation theory. The effect of filler loading on the complex and real parts of impedance was clearly observed, which can be explained on the basis of relaxation dynamics of polymer chains in the vicinity of fillers. The percolation threshold occurred near 30 phr of filler loading. Scanning electron microphotographs showed the agglomeration of the filler on and above these filler loadings. Additionally, the effect of temperature on dielectric loss tangent, dielectric permittivity, ac conductivity, and Nyquist plot of conductive black reinforced CSM vulcanizates has been studied. POLYM. COMPOS., 2010. © 2009 Society of Plastics Engineers     

Mechanical,wear, and dielectric behavior of TiO2 reinforced high-density polyethylene composites

Almost fully dense high-density polyethylene (HDPE) reinforced with submicron-sized titanium dioxide (TiO₂) ceramic filler (up to 40 vol %) was fabricated using compression molding. More than 98.5% ρ_th (theoretical density) could be obtained for all the HDPE compositions and its measured density varied between 0.94 and 2.25 g cc⁻¹. The hardness of HDPE increased considerably from 32.6 to 69 MPa (i.e., by two times) with the addition of 40 vol % TiO₂. The compression strength (19.03–34.16 MPa) and modulus of elasticity (0.49–1.05 GPa) of HDPE were also found to increase with the addition of TiO₂ filler. However, the HDPE exhibited good ductility (59% strain) up to 20 vol % TiO₂ and it was reduced with the further addition of TiO₂. The strain decreased drastically to 7.6% for HDPE-40 vol % TiO₂. Addition of TiO₂ filler leads to a considerable decrease in wear rate and coefficient of friction (COF). The wear studies revealed that the HDPE-40% TiO₂ composite exhibited a low wear rate of 1.82 × 10⁻⁵ mm³ N m⁻¹ and COF of 0.13. The dielectric constant of HDPE (at 10 kHz) was also considerably increased from 5.31 to 20.02 with the addition of TiO₂ up to 40 vol %. Achievement of such high dielectric constant for HDPE materials is the highest ever reported for HDPE. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019 , 136, 47610.     

Application of lime kiln dust as a filler in the natural rubber industry

Lime kiln dust (LKD) obtained from kraft chemical recovery systems by conversion of calcium carbonate (CaCO₃) back into calcium oxide (CaO) for reuse in the causticizing process, is mainly composed of CaCO₃. A two‐stage conventional mixing procedure was used to incorporate LKD into natural rubber (NR). For comparison purposes, four commercial fillers, stearic acid coated CaCO₃, ground CaCO₃, silica, and carbon black, were also used. The effect of these fillers on the curing characteristics and mechanical properties of NR materials at various loadings ranging from 0 to 60 phr were studied. The results indicate that the use of LKD filler resulted in a lower Mooney viscosity and shorter curing time in the NR materials. The incorporation of LKD into NR improved the Young's modulus and hardness but decreased the tensile strength and tear strength. However, LKD was better in processability than the commercial fillers. Scanning electron micrographs revealed that the morphology of the rubbers filled with reinforcing fillers, such as silica and carbon black, was finer and more homogeneous compared to the those of the rubbers filled with LKD and commercial CaCO₃. The dispersion of LKD and commercial CaCO₃ fillers in the rubber matrix was discontinuous, which in turn, generated a weak structure compared with the reinforcing fillers. According to these observations, LKD could be used as a cheaper filler for NR materials where improved mechanical properties are not critical. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

Silicone elastomers with high dielectric permittivity and high dielectric breakdown strength based on dipolar copolymers

Dielectric elastomers (DEs) are a promising new transducer technology, but high driving voltages limit their current commercial potential. One method used to lower driving voltage is to increase dielectric permittivity of the elastomer. A novel silicone elastomer system with high dielectric permittivity was prepared through the synthesis of siloxane copolymers, thereby allowing for the attachment of high dielectric permittivity molecules through copper-catalysed azide-alkyne 1,3-dipolar cycloaddition (CuAAC). The copolymers have a high degree of chemical freedom, as the dimethylsiloxane spacer units between the functional groups, as well as the degree of functionalisation, can be varied. Thus, the best overall properties were obtained for an elastomer prepared with a copolymer with a 1200 g mol⁻¹ dimethylsiloxane spacer unit and 5.6 wt% of the high dielectric permittivity molecule 1-ethynyl-4-nitrobenzene. Here, a high increase in dielectric permittivity (∼70%) was obtained without compromising other favourable DE properties such as elastic modulus, gel fraction, dielectric loss and electrical breakdown strength.     

莫来石与沉淀法白炭黑填充丁苯橡胶复合材料的性能比较

在双辊开炼机中将未处理或用质量分数为3%的硅烷偶联剂A-189处理的莫来石粉体加入到丁苯橡胶(SBR)中制成复合材料.研究莫来石的加入量对SBR性能的影响,并与沉淀法白炭黑填充的SBR的性能进行了比较.结果表明,莫来石粉体对SBR表现出半增强特性;与沉淀法白炭黑填充的SBR相比,SBR/莫来石复合材料表现出良好的硫化特...     

Improvement of thermo-electrical properties of polymer composites filled with multiwall carbon nanotubes decorated carbon fillers

To enhance the thermo-electrical properties of liquid silicone rubber (LSR) in applications, the carbon fibres (CFs) modified by multiwall carbon nanotubes (MWCNT) on the surfaces were used as the fillers. The MWCNT-modified CFs (MPCFs) were analysed by Fourier transform infrared spectra, thermogravimetric analysis, scanning electron micrograph and energy dispersive X-ray spectroscopy. It was found that MWCNT were successfully adsorbed onto the surface of CFs. The MPCFs functioned as conductive fillers in LSR for thermal and electrical conductivity application and exhibited significant enhancement. The effects of MPCFs loading on thermal conductivity and volume resistivity of LSR composites were investigated in detail. Results of this work revealed that the MPCFs/LSR composites possessed a thermal conductivity of 0.73?W?m^?1?K^?1 with 14?vol.-% filler loading, approximately 3.48-fold higher than that of pure LSR substrate. And with the increase of MPCFs loading, the least volume resistivity of MPCFs/LSR composites is 10?Ω?cm. Besides, compared with that of neat LSR, the tensile strength of MPCFs/LSR composites increased 0.913?MPa.     

Influences of porous reduction graphene oxide/molybdenum disulfide as filler on dielectric properties,thermal stability,and mechanical properties of natural rubber

A ternary composite system consisting of natural rubber (NR), porous reduced graphene oxide (rPGO), and molybdenum disulfide (MoS₂) was introduced for applying in the dielectric field, of which rPGO and MoS₂ hybrid conductive filler (rPGM) was prepared by an effective and environmentally friendly method-microwave reduction. And the well-dispersed NR composites (NGM) were made by the latex co-precipitation method. Due to the large specific surface area of rPGM itself and the synergistic dispersion of rPGO and MoS₂, it formed many stable interface structures with the NR matrix, which not only made the blend exhibit high elasticity and withstood large deformation as NR but also greatly improved the dielectric, mechanical and thermal stability of the NR matrix. Compared with neat NR, the dielectric constant of nanocomposite increased by 11 times in the presence of rPGM conductive filler, and the leakage current generated by direct contact of fillers was reduced due to the attachment of MoS₂ to the surface of rPGO; when 2% rPGM was added, the NR exhibited the highest tensile strength (21.3 MPa), elongation at break (495%), and abrasion resistance (0.165 cm⁻³); in addition, the thermal stability of the nanocomposite was also improved. These phenomena indicate that rPGM had great potential in conductive fillers and provided a reliable way for NR applications in the field of dielectric elastomers.     

Effect of high loading of titanium dioxide particles on the morphology,mechanical and thermo-mechanical properties of the natural rubber-based composites

The aim of this work was to prepare and characterize the natural rubber vulcanizates containing different amounts of titanium dioxide particles. At first, a rubber mixture was prepared using a laboratory two-roll mill and then samples were vulcanized by a hydraulic press. The formulation of the rubber mixture and rubber-processing technique were based on our earlier investigations. Samples were obtained with different titanium dioxide loadings of 15, 25, 45, and 85 parts by weight per hundred parts of natural rubber. This research is focused on the determination of the influence of different loadings of titanium oxide particles on the chemical structure, morphology, mechanical and thermo-mechanical properties of the natural rubber-based composites. It was found that vulcanizates with different amounts of TiO₂ particles possess good characteristic in terms of all measured properties. The results of Fourier transform infrared spectroscopy analysis showed that the chemical structure of the obtained natural-based composites was not influenced by titanium dioxide particles. The SEM micrographs showed the uniform dispersion of TiO₂ particles in the natural rubber matrix. The agglomeration of filler was seen at the higher contents of TiO₂ in the matrix. The thermogravimetric analysis showed slightly different thermal stability for the obtained natural rubber composites. The dynamic mechanical thermal analysis showed that the prepared materials have similar glass transition temperatures. However, increase in the content of titanium dioxide in the obtained materials is connected with higher energy loss (higher dissipation of energy) during the mechanical work of material and higher cross-link density of the prepared materials.     

Effects of metal oxides on the dielectric and mechanical properties of silicone rubber composites

Polysiloxane dielectric elastomers have garnered a considerable deal of attention over the last decade due to their potential as electroactive soft materials. However, the intrinsic low dielectric constant of polysiloxanes has proven a serious limitation in their practical use. In this work, we controlled the dielectric properties of silicone rubber composites by changing the type and content of oxide fillers. The silicone-based dielectric elastomers with a high dielectric constant (5.21@1 kHz), low modulus (1.62 MPa), and high elongation (1100%) were successfully obtained. The effects of different types of metal oxides on the dielectric properties and mechanical properties of the prepared composites are further explored. Due to their excellent comprehensive features, these types of materials are expected to be applied in capacitive sensors, actuators, generators, and beyond.     

Dielectric and microwave properties of elastomer composites loaded with carbon–silica hybrid fillers

In this study, we investigated the influence of carbon–silica dual‐phase fillers on the dielectric and microwave properties of natural‐rubber‐based composites determined in the frequency range from 1 to 12 GHz. The fillers were prepared by the impregnation of two types of carbon black with various silicasol amounts. As the results show, the fillers affected both the dielectric and microwave properties of the obtained composites. The higher the quantity of the dielectric phase (silica) in the hybrid filler was, the lower the real part of the permittivity of the composites was. This caused changes in the total, reflective, and absorptive shielding effectivenesses of the latter and their reflection and attenuation coefficients. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016 , 133, 42978.