Silicone Rubber Composites Research Articles

Constructing a highly vertically aligned network of h-BN/CF in silicone rubber composites: Achieving superior through-plane thermal conductivity and electrical insulation

Polymer composites with both high through-plane thermal conductivity (TC) and electrical insulation are desperately needed for the efficient thermal management of modern electronic devices. Herein, a highly vertically aligned network of hexagonal boron (h-BN) nitride/carbon fiber (CF) was constructed in silicone rubber composites via the combination of multistage stretching extrusion, layer-by-layer stacking and hot-pressing procedures. The composites exhibited superior through-plane TC and electrical insulation, simultaneously. The through-plane TC of the composites reached 9.5 W/(m·K) when the content of h-BN and CF was 120 phr and 5 phr, respectively. Meanwhile, the volume resistivity and breakdown voltage of the composites were 1.89 × 1013 Ω cm and 7.7 kV/mm, respectively, since the low content of CF was insufficient to form electrically conductive pathways. Furthermore, both simulation and experimental results indicated that the composites show outstanding heat dissipation performance. The composites with such hybrid networks are very promising for efficient heat dissipation in high-performance electronic devices.

Preparation and Performance of Silicone Rubber Composites Modified by Polyurethane.

Silicone rubber composites with good comprehensive properties modified with polyurethane were obtained through mixing and vulcanizing methods. Firstly, the polyurethane prepolymer with double bonds was prepared by polytetrahydrofuran glycol (PTMG, Mn = 1000), isophorone diisocyanate (IPDI), and 2-hydroxyethyl methacrylate (HEMA). The prepolymer was then added to the silicone rubber compounds to prepare silicone rubber composites, combining the excellent properties of polyurethane with the silicone rubber materials. The effects of polyurethane content on the mechanical properties, insulation, hydrophobicity, thermal stability, and flame retardancy of composites were studied in detail. The results showed that the silicone rubber composites not only have good hydrophobicity, thermal stability and flame retardant properties, but the addition of polyurethane significantly improves the tensile strength at room and low temperatures and the volume resistivity of the materials. The tensile strength increased by 32.5%, and the volume resistivity nearly doubled. The excellent electrical insulation, high hydrophobicity and good mechanical properties make the silicone rubber composites appropriate for use in the field of polymeric house arresters.

Enhanced Dielectric and Mechanical Performances of the (La + Nb) Codoped TiO2/Silicone Rubber Composites

Silicone rubber (SR) has gained great interest because of its fast deformation response, low modulus, high rebound rate, and wide temperature application range. However, their development is hampered by the low dielectric constant. Adding dielectric ceramics may be a solution. Herein, (La + Nb) codoped TiO2/SR composites are fabricated and their properties are systematically tested. By adding codoped TiO2, the performances of the composites are enhanced. High permittivity (3.63) and low loss tangent (7.07 × 10−4) are gained in the SR composites with 30 wt% codoped TiO2. The evenly dispersed codoped TiO2 and the enhanced interfacial polarization effect can be responsible for this. The composites also exhibit enhanced tensile strength (565 kPa) and elongation (259%). In addition, the SR composites are thermally stable up to 415 °C. These superior properties can improve the electrodeformation performance of SR composites, which has great potential in the fields of flexible actuators, bionic flexible robots, next‐generation intelligent medical devices, and green energy harvesting.

Effect of spherical alumina crystalline phase content and particle size distribution polydispersity on the properties of silicone rubber composites

High loading of thermally conductive fillers for enhancing thermal conductivity (TC) conflicts with proper rheological viscosity in excellent thermal interface materials (TIMs), which remains a significant challenge. The physical properties (crystalline phase, particle size and distribution, etc.) of thermally conductive filler are the base of the high load in the polymer. In this paper, the α-phase content of spherical aluminum (S–Al2O3) increases after calcination, leading to the viscosity of composites increasing sharply but was only slightly improved for the TC. The polydispersity of the S–Al2O3 particle size distribution was regulated to fill the silicone rubber under the premise of controlling the same average particle size, crystalline phase content and filling amount. Results indicated the viscosity of the sample with a polydispersity value of 0.74 was 41.21 Pa s at a shear rate of 10 s−1 at 66 vol%, which was 87.7% lower than that of the sample with a polydispersity value of 0.48. And the viscosity difference became more prominent with the increasing filling amount. However, the difference in TC between several groups of samples with different polydispersity is insignificant at 60–67 vol%, which provides practical, theoretical guidance for achieving controllable viscosity and developing low-viscosity TIMs in the processing process.

The effect of modified carbon-doped boron nitride on the mechanical, thermal and γ-radiation stability of silicone rubber composites

The developing nuclear technology has increased the need for radiation-resistant and shielding materials. It is crucial to study the radiation resistance of polymers in order to prevent safety incidents brought on by the degradation of polymer performance in a nuclear environment. In this work, γ-aminopropyltriethoxy silane (KH550) was successfully grafted onto carbon-doped boron (BCN) to obtain modified BCN (BCN-KH550), then BCN-KH550 was added in silicone rubber and refined and vulcanized. The elongation at break of silicone rubber composite doped with 0.3 phr BCN-KH550 can retain (54 ± 2)% of the initial value. The addition of BCN and BCN-KH550 both increases the initial decomposition temperature of the irradiated SR. The radiation caused changes in crystallinity and glass transition temperature (Tg) were measured using differential scanning calorimetry. The difference Tg of BCN-KH550/SR0.3 before and after irradiation is 0.9 °C higher than that of SR. It is supposed that BCN protects the side group of SR during irradiation through differential thermogravimetric analysis curve and gas phase infrared spectra of the irradiation-induced gas products. BCN and BCN-KH550 can lower the hydrocarbon, CO2 and CO production by side chain breakdown in both air and nitrogen atmospheres. The presence of BCN and BCN-KH550 decrease the radiation crosslinking of SR. The outcomes demonstrate that BCN-KH550 can reduce the effects of γ-radiation crosslinking on SR by declining side chain decomposition.

Al@SiO2 Core-Shell Fillers Enhance Dielectric Properties of Silicone Composites.

Over the past decade, there has been significant interest in polysiloxane-based dielectric elastomers as promising soft electroactive materials. Nevertheless, the natural low permittivity of polydimethylsiloxane has limited its practical applications. In this study, we have developed silicone rubber/Al@SiO2 composites with a high dielectric constant, low dielectric loss, and high electrical breakdown strength by controlling the shell layer thickness and the content of the core-shell filler. We also investigated the dielectric behavior of the composites. The use of core-shell fillers has increased the Maxwell-Wagner-Sillars (MWS) relaxation process while reducing the dielectric loss of direct current conductance in silicone rubber composites. Moreover, the temperature dependence of the MWS relaxation time in the composites follows the Arrhenius equation. This strategy of increasing the permittivity of silicone composites through core-shell structural fillers can inspire the preparation of other high dielectric constant composites.

Conformal BN coating to enhance the electrical insulation and thermal conductivity of spherical graphite fillers for electronic encapsulation field

Herein, we provided a structural design strategy of constructing boron nitride (BN) conformal coating on the surface of the spherical graphite (SG) particles via precursors deposition and high-temperature pyrolysis method to achieve improved electrical insulation. The dissolved and reprecipitated BN precursors were uniformly deposited on the surface of polydopamine-modified SG through strong adhesion and hydrogen bonding of polydopamine. Then, the homogeneous and dense BN ceramic coating was formed on the surface of SG via slow pyrolysis at high temperature. The complete encapsulation of BN coating prevents the SG particles directly contacting with each other in the silicon rubber (SR) matrix to retain electrical insulation of SR composites. Furthermore, the BN coating flattens the uneven surface of SG so that the fillers are better infiltrated by the SR matrix. The small specific surface area of SG@BN filler particles is beneficial to significantly reduce the generation of the filler/matrix interfaces and weaken phonons scattering, thus decreasing the thermal boundary resistance. As a result, the thermal conductivity of prepared SG@BN/SR composites is up to 3.65 W m−1 K−1, which is 20.2-fold of that of pure SR with 0.18 W m−1 K−1. And the SG@BN/SR composites exhibit a satisfactory breakdown voltage of 5.6 kV mm−1, which is much higher than 1.1 kV mm−1 for the original SG/SR composites. This strategy is worth using for reference to extend the application of graphite-type fillers in the electronic encapsulation field.

In-situ constructing SiC frame in silicone rubber to fabricate novel flexible thermal protective polymeric materials with excellent ablation resistance

To meet the urgent need for thermal protection of high-temperature deformable mechanisms in the aerospace industry, a novel strategy for the preparation of ablation-resistant flexible composites is proposed. Ceramic precursor is selected and introduced to epoxy modified silicone rubber (SR) to in-situ build SiC frame at high temperature to fabricate flexible thermal protective polymeric materials with excellent ablation resistance. The static ablation and dynamic ablation behaviors of the SR composites are systematically and comparatively investigated in a tube furnace and oxyacetylene torch in an automatic ablation machine for the first time. The influence of polycarbonylsilane (PCS) content on the microstructure, strength and composition of char layer, and the ablation properties at different heat environments are discussed. When the addition of PCS reached 10 phr, for the static ablation, the maximum puncture strength of char and the residual weight increased 3233% and 48.48%, respectively. The same for dynamic ablation, with the introduction of PCS, each indicator was greatly improved. Moreover, the ablation enhancement mechanisms of PCS in different ablation environments are revealed. This work may provide a new perspective for the preparation of silicone rubber composites with excellent ablation performance, expected to be well used as a thermal protection coating for hypersonic spacecrafts.

A novel application of titanium suboxide to enhance the flame retardancy, thermal stability, and thermal and electrical conductivity of silicone rubber

This study investigates the impact of Titanium Suboxide (Ti4O7) on the flame retardancy, mechanical properties, thermal stability, and electrical and thermal conductivity of silicone rubber composites. The addition of 1.0 part of Ti4O7 to the silicone rubber composites results in a total heat release (THR) of 45 MJ/m2, which is 45.7% lower than the silicone rubber without Ti4O7, leading to a decrease in the peak of heat release rate (PHRR). Furthermore, Ti4O7 exhibits temperature stability at low temperatures and acts as a physical barrier, inhibiting the early decomposition of silicone rubber and enhancing the thermal stability of the composites. However, at temperatures exceeding 450°C, the catalytic activity of Ti4O7 is activated, promoting the decomposition of the silicone rubber composites and generating more residue. The addition of an appropriate amount of Ti4O7 can further improve the mechanical properties of the silicone rubber composites and enhance their electrical and thermal conductivity.

Intercalation of silsesquioxane surfactant in layered double hydroxide for silicone rubber composites: Enhancing interfacial interactions toward improvement of mechanical, thermal, and gas barrier properties

AbstractThe incorporation of two‐dimensional layered metal hydroxides (LDHs) into nonpolar silicone rubber (SR) is not yet universal, mainly because of their originally hydrophilic surface and limited interlamellar galleries that make nanoparticle dispersion and intercalation of polymer segments more difficult. In this work, we develop a modified Ca‐Mg‐Al LDH form by conjugating the ternary Ca‐Mg‐Al LDH with silsesquioxane (SQ) surfactants containing benzoic acid moieties and polyethylene glycol chains via an in‐situ intercalation process. The bulky anions conjugation to the hydroxide layers dictated both the surface hydrophobicity and interlayer spacing of the modified Ca‐Mg‐Al LDH, enabling strong inter‐constituent interactions within the nanocomposites as well as extensive polymer penetration after incorporation into the SR matrix. The SR/modified LDH nanocomposites show excellent thermal stability and satisfactory mechanical properties due to the homogeneous dispersion and good compatibility of modified Ca‐Mg‐Al LDH. Moreover, we have observed that the shear‐induced orientation of the nano‐layered particles upon application of stress imparts a more notably decreased SR/LDH‐SQ‐40 gas transmission rate (reduced by 55% with respect to that in the unstretched composite films). This result also corroborates the better interfacial bonding and highly efficient load transfer between silsesquioxane‐modified LDH and polymer in melt extrusion mixing of such composites.Highlights Modified Ca‐Mg‐Al LDH with increased layer spacing and reduced hydrophilicity. Modifier enhanced the interfacial adhesion between Ca‐Mg‐Al LDH and SR matrix. SR/modified LDH showed excellent thermal stability and mechanical properties. Orientation and exfoliation enhanced gas barrier properties of SR/modified LDH.

The Thermal Stability, Ablation Properties, Creamization Mechanism of Graphite/Zirconia (ZrO2)/Silicone Rubber Composites Containing Carbon Multicomponent Systems

It is urgent to improve the long-term ablation resistance of silicone rubber composites in order to meet the application requirements in the field of aerospace thermal protection. The graphite/ZrO2/silicone rubber (G/ZrO2/SR) composites prepared in this study were for that purpose. Firstly, the thermal stability and ablation properties of the G/ZrO2/SR composites were studied. The results showed that with the increase of graphite content, the initial decomposition temperature of the composites increased from 456.5 °C to 478.6 °C. The linear and mass ablation rates of the G/ZrO2/SR composite with 3 phr graphite were 0.048 mm/s and 0.0,220 g/s, respectively, which were decreased by 14.3% and 9.1%, compared with the ZrO2/SR composite without graphite. Moreover, the phase composition transformation and ceramization mechanism at high temperature of the G/ZrO2/SR composites containing carbon multicomponent system were studied. The results showed that the SR matrix completed the ceramic transformation during the ablation process and -Si-O-C pyrolysis products were formed and served as the skeleton of the ceramic layers. Finally, SiO2 and ZrO2 reacted with the graphite individually and the in-situ SiC and ZrC ceramic products formed enhanced the ceramic layers.

Optimizing electromechanical performance of multilayered multiwalled carbon nanotube/silicone rubber composites via filler content distribution modulation

AbstractDielectric elastomers are widely used as electroactive polymer materials due to their high energy density, high strain, and low loss. However, the practical applications are limited sometimes by their low electromechanical strain ability due to relatively low dielectric permittivity for polymers. Herein, multiwalled carbon nanotube (MWCNT) filled silicone rubber (SR) composites with a five‐layered structure are prepared with the outer two layers of neat SR while the middle three layers are MWCNT filled SR, in which the former functions as insulating layers while the latter acts as the dielectric permittivity enhancement layers. Further, by differing the MWCNT content within the middle three layers, we find that as the concentration distribution increases from 1:1:1 to 1:2:1 and 1:3:1, both Young's modulus and dielectric permittivity gradually improve while dielectric loss remains extremely low even though the total MWCNT content reaches 1.6 wt%. With the combined effects of dielectric permittivity and modulus, the composite with evenly distributed MWCNT content (1:1:1) shows the highest actuation strain under a given electric field strength. Meanwhile, the electric breakdown strength of the composite 1:1:1 is also the highest, leading ultimately to a maximum actuation strain of 10.87% at the breakdown strength of 14.6 kV/mm.

Effect of Fluorosilicone Rubber on Mechanical Properties, Dielectric Breakdown Strength and Hydrophobicity of Methyl Vinyl Silicone Rubber.

Silicone rubber (SIR) is used in high-voltage insulators because of its insulation, and excellent hydrophobicity is very important in harsh outdoor environments. To enhance the hydrophobicity and low-temperature resistance of silicone rubber, methyl vinyl silicone rubber and fluorosilicone rubber (FSIR) blend composites with different ratios were prepared. The samples were characterized and analyzed using scanning electron microscopy, tensile testing, dynamic mechanical analysis and static contact angle testing. The results showed that after blending, SIR and FSIR were well compatible. FSIR had higher elastic modulus and reduced the tensile strength to some extent in SIR/FSIR composites. The addition of a small amount of FSIR made its crystallization temperature decrease from -30 to -45 °C, meaning that the low-temperature resistance was significantly improved. The breakdown strength of SIR/FSIR composites can still be maintained at a high level when a small amount of FSIR is added. The contact angle of the composites increased from 108.9 to 115.8° with the increase in FSIR content, indicating the enhanced hydrophobicity. When the samples were immersed in water for 96 h, the hydrophobicity migration phenomenon occurred. The static contact angle of the samples with less FSIR content had a weaker decreasing trend, which illustrated that the hydrophobicity was maintained at a high level.

Effect of Modified Nanographene Oxide (mGO)/Carbon Nanotubes (CNTs) Hybrid Filler on the Cure, Mechanical and Swelling Properties of Silicone Rubber Composites

Effect of Modified Nanographene Oxide (mGO)/Carbon Nanotubes (CNTs) Hybrid Filler on the Cure, Mechanical and Swelling Properties of Silicone Rubber Composites

High ablation-resistant silicone rubber composites via nanoscale phenolic resin dispersion

High ablation-resistant silicone rubber composites exhibit significant potential for solid rocket engine ablation coating applications. However, the low char yield of silicone rubber restricts its real-world usage. Herein, silicone rubber with nanoscale dispersed Novolac-type phenolic resin particles (novolac-silicone rubber, NSR) is synthesized. The introduction of dynamic urethane bonds is crucial for achieving nanoscale dispersion of the phenolic resin, as it leads to both improved mechanical strength and enhanced char yield. In addition, the NSR-based composite materials demonstrate linear and mass ablation rates of 0.1440 mm/s and 0.0466 g/s, respectively, representing reductions of 44.19 % and 17.81 % compared with their conventional silicone rubber counterparts. Moreover, unlike the traditional physical blending approach, the nanoscale dispersion in this work is achieved by introducing a dihydroxypropyl-terminated silicone oil-novolac phenolic resin block copolymer to the silicone rubber matrix. The enhanced structural compatibility is proven to be pivotal for the improved performance of NSR. This work provides new routes for the synergistic improvement of the ablation resistance and mechanical properties of silicone rubber composites.

Synergistic Effects between Multiphase Thermal Insulation Functional Phases on the Mechanical and Heat Insulation Properties of Silicone Rubber Composites.

This work utilizes the synergistic effect between three different functional phases of thermal insulation, i.e., hollow ceramic microspheres (HCMs), hollow silica microspheres (HSMs), and hydroxy silicone oil blowing agent, to prepare a flexible and efficient thermal insulation composite with low thermal conductivity and high structural strength. First, the effects of the three phases on the mechanical and thermomechanical properties of silicon rubber (SR) were analyzed using a scanning electron microscope (SEM), a thermogravimetric (TG) analyzer, a thermal conductivity meter, and a universal testing machine, respectively. Then, the thermal insulation mechanism of multiphase thermal insulation composite materials was analyzed. The results show that the thermal stability and mechanical performance of composites were significantly enhanced, particularly for sample 18H, whose Tmax and char yield reached 621.3 °C and 77.5%, respectively, representing a respective increase of 12.1 and 35.3% compared to those of pure SR. After heat treatment at 1000 °C, the linear shrinkage of the sample diameter was about 9.4%, while the thermal conductivity was as low as 0.064 W/(m·K), which was 53.2% lower than that of the pure matrix SR. We believe that this work can serve as a reference for the preparation of heat insulation and protection materials with low thermal conductivity and high structural strength.

Waffle-inspired thermal conductive silicone rubber composites with excellent electromagnetic shielding performance based on three-dimensional filler networks

Improving the thermal conductivity and electromagnetic shielding performance of silicone rubber composites is crucial for their application in electronic devices. In this work, inspired by the structure of waffle, a novel approach combining ice templating and mechanical vibration was employed to prepare reduced graphene oxide (rGO)/silicon carbide whisker (SiC) filler networks arranged in both horizontal and vertical directions. Silicone rubber composites were prepared by vacuum impregnation of silicone rubber into the three-dimensional filler networks. At the hybrid filler content of 3.88 vol%, the through-plane thermal conductivity reached 5.24 W/(m·K), exhibiting about 31 times higher than that of silicone rubber, and the anisotropy of thermal conductivity of the composite was only 1.35 due to the structural arrangement of the filler network in both vertical and horizontal directions. The total electromagnetic interference (EMI) shielding effectiveness of the composite with 3.88 vol% hybrid filler was 35.2 dB with low electromagnetic reflection, exhibiting good EMI shielding performance.

Radiation synthesis of boronic acid modified graphene oxide for enhance the neutron shielding property of silicone rubber

In this study, poly(3-acrylaminophenylboronic acid) modified graphene oxide (GO-PAAPBA) has been prepared by polymerization of 3-acrylaminophenylboronic acid (AAPBA) on the surface of GO. The –O· free radical was formed by γ-ray radiation, which initiated the polymerization of AAPBA. Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy and Raman spectroscopy were employed to characterize the structure of GO-PAAPBA. The grafting density of poly(3-acrylaminophenylboronic acid) is increased with increasing of the absorbed dose. The GO-PAAPBA was added into vinyl-terminated polydimethylsiloxane (Vi-PDMS) to prepare room temperature cured silicon rubber composites (SR/GO-PAAPBA). The GO-PAAPBA exhibits better dispersion performance than pure GO, which is contribute to improve thermal conductivity and mechanical properties. Significantly, with grafting density of PAAPBA increasing, the comprehensive performances of SR/GO-PAAPBA are more remarkable. SR/GO-PAAPBA-1.88 with outstanding thermal and mechanical abilities exhibits beat neutron shielding property. The neutron transmission of SR/GO-PAAPBA-1.88 is 10% lower than that of SR. The SR nanocomposite with efficient thermal conductivity and neutron shielding ability has a promising prospect to be applied as advanced flexible shielding material.

Significantly improved ablation properties under high heat flux environment based on carbon fiber reinforced Zr-doped silicone rubber

In order to improve the ablative properties of silicone-rubber (SR) based ablative coatings under high heat flow, firm char layer required to be formed during ablation. Herein, Zr-doped SR composites with significantly improved ablation properties were prepared by co-blending synthesized zirconium-doped polydimethylsiloxane and short-cut carbon fibers (Zr-PDMS-CF). Compared to carbon fiber modified pure polydimethylsiloxane (PDMS-CF) and ZrO2-modified PDMS-CF (ZrO2 @PDMS-CF), the mass ablation rates were decreased by 28.96% and 28.53%, respectively, and the linear ablation rates were decreased by 47.88% and 38.20%, respectively. Compared to PDMS-CF and ZrO2 @PDMS-CF, the compressive strength of char layer of Zr-PDMS-CF was also significantly enhanced. In addition to the well distribution of t-ZrO2 crystals in the char layer of Zr-PDMS-CF, the homogeneous distribution of zirconium elements was also beneficial to protect CFs skeleton from severe oxidation. The char layer of Zr-PDMS-CF thus possessed the ability to resist erosion under high heat flow, protecting underneath materials from further oxidation and erosion.

Enhancement of nuclear radiation shielding properties of silicone rubber with γ‐ray irradiation induced graphene oxide modified carbonyl iron powder hybrid fillers

AbstractIn this study the graphene oxide modified carbonyl iron (GO‐CI) was prepared by γ‐ray irradiation, which was added to silicone rubber composites to enhance the nuclear radiation shielding performances. X‐ray diffraction (XRD) characterization result showed the structure and morphology of the obtained GO‐CI nanocomposites. The mixture of GO/CI and the hybrid GO‐CI were individually incorporated into the vinyl‐terminated polydimethylsiloxane (Vi‐PDMS) to prepare referenced SR/GO/CI and SR/GO‐CI composites. Compared with SR/GO/CI, the SR/GO‐CI exhibited better thermal conduction and mechanical recovery properties. Moreover, the γ‐ray radiation shielding ability of SR/GO‐CI is much higher than that of pristine SR/GO/CI. CI particles are coated by GO nanosheets to increase surface area to prevent sedimentation of CI. It implies that SR‐GO/CI composite would have potential application as the radiation hardening material with thermal conductive and nuclear radiation shielding function.