Liquid Silicone Rubber Research Articles

Synthesis and application of adhesion promoter containing phenolic hydroxyl/acrylate groups for addition-cure liquid silicone rubber

Grafting polar polysiloxanes to addition-cure liquid silicone rubber (ALSR) is a promising method to improve its adhesion performance. Herein, we synthesized a novel silicone adhesion promoter containing phenolic hydroxyl and acrylate groups via hydrosilylation reaction, denoted as PMHS-g-(EE/TD). It was found that PMHS-g-(EE/TD) effectively enhanced the bond strength of ALSR with polycarbonate (PC) and polyamide (PA) substrates. When the PMHS-g-(EE/TD) content was 2 phr, the bond strength of ALSR with PC and PA substrates were 3.8 MPa and 3.1 MPa, respectively. More importantly, after being heat-treated at 150 °C for 72 h or soaked in 25 °C distilled water for 8 h, the bond strength of ALSR/PMHS-g-(EE/TD) with PC and PA substrates maintained a rather high value. The adhesion promotion mechanism of PMHS-g-(EE/TD) for ALSR was further discussed and revealed. On the one hand, PMHS-g-(EE/TD) participated in the curing process of ALSR through CC bonds and formed chemical bond connections with the ALSR chain; on the other hand, PMHS-g-(EE/TD) was enriched on the ALSR surface to establish a boundary layer containing –OH and –COO, forming hydrogen bonds with the substrate. PMHS-g-(EE/TD) acted as a bridge to tightly connect the substrate and ALSR, which greatly improved the adhesion performance. This work provides a simple approach to fabricating ALSR with prominent adhesive properties.

Preparation of thermally conductive flexible liquid silicone rubber composites by magnetic field‐induced self‐assembly of Fe3O4@C

AbstractEfficient heat dissipation is very crucial for the performance, service life, and reliability of modern electronic devices. It is very challenging to improve heat dissipation efficiency through innovative material structure design. Therefore, in the work, dopamine (PDA) is used to coat the surface of graphite (C) with its excellent surface coating modification ability, so that the C surface has oxygen‐containing groups to change its electronegativity, and Fe2+ and Fe3+ in the solution are attracted to the surface of C, and then in situ redox reaction occurs on the surface of C to generate magnetic graphite (Fe3O4@C). Fe3O4@C is uniformly dispersed into liquid silicone rubber (LSR) by solution blending, and Fe3O4@C performs integral directional filling in the LSR under the action of magnetic field (B = 140 mT), thus preparing Fe3O4@C/LSR composites. Through magnetic field orientation, the thermal conductivity of Fe3O4@C/LSR composites reach 1.25 W/m·K when the content of Fe3O4@C is 100 phr, which is 410% higher than that of pure LSR (0.2447 W/m·K). Infrared thermal imaging test shows that Fe3O4@C/LSR composites has good heat dissipation performance, Fe3O4@C/LSR composites have shown great potential in terms of flexibility and high thermal conductivity.

Processing, thermal conductivity and flame retardant properties of silicone rubber filled with different geometries of thermally conductive fillers: A comparative study

With the development of highly integrated and miniaturized electronic devices, silicone rubber composites with good mechanical flexibility, high thermal conductivity and excellent flame resistance have aroused considerable interest in both industrial and research community. However, the effect of different geometries of thermally conductive fillers on the processing, mechanical and thermal properties as well as flame resistance of silicone rubber composites is still not clear. Herein, liquid silicone rubber (LSR) composites filled with three geometries of thermal conductive fillers i.e. spherical-like aluminum oxide (Al 2 O 3 ), irregular aluminum nitride (AlN) and 2D boron nitride (BN) sheets were prepared by an extremely simple mechanical-mixing process. As expected, incorporation of BN sheets into LSR matrix produces much higher increase in the viscosity, tensile strength and hardness than the corresponding AlN and Al 2 O 3 fillers, respectively. Typically, at a fixed filler loading of 120 phr, the increased thermal conductivity and heating/cooling efficiency of the BN/LSR composites display much better than the corresponding AlN/LSR and Al 2 O 3 /LSR composites, e.g. 316, 124 and 88% improvement in thermal conductivity, respectively. Interestingly, the thermal stability and flame resistance of the three composites present unexpected different phenomena. Compared with the pure LSR, the addition of AlN and Al 2 O 3 exacerbates the thermal degradation of LSR chains, while the presence of BN sheets improves the flame resistance of LSR dramatically, which is attributed to the compact BN/silica residue after burning that effectively restricts the heat and oxygen to attack the inside LSR chains.

Breakdown Characteristics of Liquid Silicone Rubber in Needle-Plane Configuration Under Polarity Reversal

The results of breakdown voltages in liquid silicone rubber (LSiR) insulation under stepped polarity reversal voltages at a uniform temperature in a needle-plane electrode system are presented in this article. In the presence of defects such as sharp conductor projections or protrusions in the cable insulation stressed under polarity reversal direct current (dc) voltages, the electric field distribution is far more complex due to nonlinear insulation conductivity and time-dependent field distribution. The electric field distribution and space-charge accumulation are intricate functions of factors such as geometry and nonlinear conduction. In this article, a two-dimensional (2-D) axisymmetric model of a needle-plane geometry is developed using the finite-element method to compute the time-dependent field, space charge, and current density distribution under polarity reversal dc, which apparently did not get much attention until now. Interesting results on the effect of polarity transition time and applied polarity time step on the abovementioned distributions are addressed and presented. The pronounced effects of nonlinear conductivity on the time-dependent field distribution are also demonstrated. Furthermore, from the breakdown experiments conducted on tip-plane electrode system for different tip radii and applied time steps, the results put forward a rational and practical estimate of the breakdown field using the proposed model.

Liquid Silicone Rubber Foamed with Thermoplastic Expandable Microspheres.

To reduce the material costs as well as the density of Liquid Silicone Rubber (LSR), LSR foams can be produced in an injection molding process. Expandable thermoplastic microspheres can be used as blowing agents. This publication deals with the analysis of the cell structure of these LSR foams. For this purpose, cylindrical disks are injection molded and examined for their cell structure as a function of different proportions of microspheres using a scanning electron spectroscope. In addition, the density of the samples is determined. It was found that a very homogeneous cell structure is produced in this process, that heat transport has a significant influence on the expansion of the microspheres, and that the formation of a filler network limits the expansion at higher proportions of blowing agent.

Highly integrated, scalable manufacturing and stretchable conductive core/shell fibers for strain sensing and self-powered smart textiles

Although the research in triboelectric nanogenerator (TENG) textiles has seen a rapid development recently, their integrated and mass fabrication process is still challenging, which hinders its further applications for wearable sensors. Herein, highly integrated and scalable manufacturing conductive composite fibers for weaving TENGs are presented, which might overcome the major problems. The fibers possess liquid alloy/silicone rubber core/shell structures made by simultaneously injecting liquid alloy and silicone rubber into the separate input ports of a coaxial needle, followed by automatically assembled from the output. The liquid alloy/silicone rubber core/shell fiber (LCF) has both good pliability and high resistance-strain sensitivity, which is beneficial for serving as strain sensors directly, and for incorporating with woven for textile-TENGs (t-TENGs)-based self-powered sensoring application. As a result, the open-circuit voltage (Voc), short-circuit current (Isc), short-circuit transferred charge (Qsc) and maximum power density of 4 × 4 cm2 t-TENG are 175 V, 15 μA, 66 nC and 469 mW/m2, respectively. Additionally, the t-TENG is mechanically robust, chemically stable and easy-cleaning for daily use. The wearable t-TENG devices can be used to detect human motions. This work provides a novel method of scalable manufacturing LCFs for weaving wearable t-TENGs, contributing to the development of t-TENGs and wearable self-powered sensors.

Calibration Dependencies and Accuracy Assessment of a Silicone Rubber 3D Printer

Silicone rubbers are relatively new in additive manufacturing, with only a few commercial printing services and reports on custom-built printers available. Publications and standards on calibration and accuracy assessment are especially lacking. In this study, the printhead calibration process of a custom-built silicone printer is explained, and a set of test objects is proposed and evaluated. The printer in use is based on an open-source filament printer, capable of multi-material printing with silicone rubbers and thermoplastic polymers. Three different high-viscosity single-component liquid silicone rubbers and one polylactic acid thermoplastic filament were used as printing materials. First, the calibration process of the silicone printhead was conducted, and the dependency of the dosing accuracy on silicone viscosity, nozzle diameter and extrusion speed was evaluated. Second, various test specimens were proposed and printed to characterize the accuracy and geometric limitations of this printer. These test parts contained features such as thin walls, slender towers, small holes and slots, unsupported overhangs and bridges. It was concluded that silicone viscosity strongly affects geometric inaccuracies. Design recommendations were deducted from the results, advising for wall thicknesses above 1 mm, slenderness ratios below 2, bridging lengths below 2 mm and unsupported overhang angles below 30°.

Enhanced ablation resistance of Divinyl‐POSS modified additional liquid silicone rubber and its fiber reinforced composite

AbstractTo further improve the ablation and thermal properties of silicon rubber composite materials, the 3, 13‐divinyloctaphenyl double‐decker polyhedral oligomeric silsesquioxane (DV‐DDSQ) modified additional vinyl‐terminated polydimethylsiloxane (VPDMS) was prepared, and the effect of high silica fiber reinforcement on the ablation properties of VPDMS was investigated. The results show that when 0.8phr DV‐DDSQ was added, the mass residual rate at 800°C of the composites increased from 40.6% to 51.0%. The linear ablation rate (LAR) as well as mass ablation rate (MAR) of the composites of the DV‐DDSQ modified VPDMS, the high silica fiber reinforced VPDMS composite (HSF/VPDMS composite) and the high silica fiber reinforced DV‐DDSQ modified VPDMS composite (HSF/DV‐DDSQ‐VPDMS composite) decreased dramatically compared with raw VPDMS, and it was 16.36%, 35.69%, and 37.17% bring down for LAR and 5.34%, 14.60% and 25.59% for MAR, respectively. The tensile strength and hardness of the DV‐DDSQ modified composites increased by 9.52% and 14.13%, respectively, while the elongation at break decreased by 5.99%, compared with the raw VPDMS.

Cyclic Compression Testing of Three Elastomer Types-A Thermoplastic Vulcanizate Elastomer, a Liquid Silicone Rubber and Two Ethylene-Propylene-Diene Rubbers.

Thermoplastic elastomer vulcanizate (TPV) and liquid silicone rubber (LSR) are replacement candidates for ethylene-propylene-diene rubbers (EPDM), as they offer the possibility for two-component injection moulding. In this study, these material types were compared side by side in cyclic compression tests. The materials were also characterized to provide details on the formulations. Compared to the rubbers, the TPV had higher compression set (after a given cycle) and hysteresis loss, and a stronger Mullins effect. This is due to the thermoplastic matrix in the TPV. The LSR had lower compression set (after a given cycle) than the EPDM, but stronger Mullins effect and higher relative hysteresis loss. These differences between the LSR and the EPDM are likely due to differences in polymer network structure and type of filler. Methods for quantifying the Mullins effect are proposed, and correlations between a Mullins index and parameters such as compression set are discussed. The EPDMs showed a distinct trend in compression set, relative hysteresis loss and relaxed stress fraction vs. strain amplitude; these entities were almost independent of strain amplitude in the range 15–35%, while they increased in this range for the TPV and the LSR. The difference between the compression set values of the LSR and the EPDM decreased with increasing strain amplitude and increasing strain recovery time.

Effects of Curing Temperature and Adhesion Promoters on Adhesion of Addition-cure Liquid Silicone Rubber

Effects of Curing Temperature and Adhesion Promoters on Adhesion of Addition-cure Liquid Silicone Rubber

Vertically aligned carbon nanotubes/graphene/cellulose nanofiber networks for enhancing electrical conductivity and piezoresistivity of silicone rubber composites

The directional arrangement of a conductive filler is an effective method to improve electrical conductivity of composites. In this paper, aqueous dispersions of carbon nanotubes and graphene were prepared by high-pressure microfluidization technology. After mixing cellulose nanofiber into these dispersions, vertically aligned carbon nanotubes/graphene network foams were prepared using an ice-template assembly method, to which liquid silicone rubber was filled to obtain carbon nanotubes/graphene/silicone rubber composites. Polymethylhydrosiloxane was grafted with 3-(trimethoxysilyl) propyl methacrylate and the resultant copolymer was used as a coupling agent to improve interface interaction between silicone rubber and filler. With increasing carbon nanotubes content, the electrical conductivities of the foams and composites increased. The composite prepared with 4 g CNT and 1 g graphene aqueous dispersions had the conductive percolation threshold of 0.08% and exhibited high conductivity at low filler content. The addition of the copolymer increased the compressive strength of the composites. The composite prepared with 1 g CNT and 4 g graphene aqueous dispersions was encapsulated in an insole as a piezoresistor to effectively detect the number of pedestrian steps, and had a stable output of electrical signal. This work provides a method for preparing conductive silicone rubber composites with low conductive percolation threshold and excellent piezoresistivity.

The Effect of Phenyl Content on the Liquid Crystal-Based Organosilicone Elastomers with Mechanical Adaptability.

An elastomer with mechanical adaptability is a new kind of polymer material in which the increasing stress under continuous deformation is significantly inhibited in a large deformation area. Liquid crystal-based organosilicone elastomers, which can dissipate energy through reversible internal phase transition under external stimulation and have recoverable large deformation capacity, have drawn much interest as mechanical adaptability materials. However, there is no good way to control the mechanical adaptability at present. For this purpose, we prepared a new liquid crystal-based phenyl silicone rubber (LCMVPQ) using two-step click reactions and systematically explored the effect of phenyl content on its mechanical adaptability to achieve the regulation of mechanical adaptability. With an increase in phenyl content in the LCMVPQs, phenyl can hinder the rearrangement of the mesogenic units along the applied stress direction, which enables the adjustment of mechanical adaptability to meet the needs of different situations. In addition, the introduction of the liquid crystal phase impedes the internal friction of the molecular chain movement of the LCMVPQs and reduces the damping performance of silicone rubber. This research achieves the regulation of elastomers with mechanical adaptability and is expected to be applied in practical application fields.

Design and analysis of watch speaker to enhance waterproof performance by using liquid silicone rubber side diaphragm

Smartwatches provide many useful functions to users, such as texting messages, receiving social media updates, and playing music. The microspeaker used in a smartwatch needs to be smaller than that used in a smartphone, and it also requires high-quality acoustic and good waterproof performance. This study proposes a watch speaker with improved low-frequency acoustic performance by using the electromagnetic-mechanical-acoustic (E-M-A) coupling analysis method. Experimental results obtained using a prototype were used to validate the analysis method. Based on the same method, a novel type of watch speaker with smaller airgaps demonstrated an improved sound pressure level (SPL) at low frequencies when using a 60 A shore-hardness liquid silicone rubber (LSR) side diaphragm (SDP). The novel type using 60 A hardness SDP is named novel type #1. At the same time, novel type #2 and novel type #3 were analyzed for comparison with novel type #1. The analysis SPL results show that the novel type #2 shows a better low-frequency SPL and a minor drop in the mid-frequency. The watch-speaker samples were manufactured and subjected to SPL measurements. Furthermore, the novel type #2 with LSR has lower von Mises stress on the SDP, excellent waterproof performance that can withstand 10 ATM, and similar total harmonic distortion (THD) to prototype.

Promoters for Improved Adhesion Strength between Addition-Cured Liquid Silicone Rubber and Low-Melting-Point Thermoplastic Polyurethanes.

A polydimethylsiloxane armed with epoxy, alkoxy and acrylate groups was synthesized from silanol terminated-PDMS and epoxy and acrylate groups functionalized silane coupling agents, and utilized as the adhesion promoter (AP) to prepare addition-cured liquid silicone rubber that exhibited self-adhesion ability (SA-LSR) with biocompatible thermoplastic polyurethanes (TPU) sheets. The structural characteristics of AP were characterized by Fourier transform infrared (FTIR) spectroscopy, which demonstrated the strong adhesion to polyester-based TPU sheets due to a sufficient amount of acrylate groups, epoxy groups and silanol groups obtained by the hydrolysis of alkoxy groups. In detail, the peel-off strength of SA-LSR and TPU joints reached up to 7.63 N mm−1 after the optimization of adhesion promoter including type and content, and curing condition including time and temperature. The cohesive failure was achieved during the sample breakage process. Moreover, the SA-LSR showed a good storage stability under proper storage conditions. This design strategy provided the feasibility to combine the advantages of addition-cured liquid silicone rubber and plastics with low melting points, promoting the potential application range of those silicone-based materials.

Correction to “Ablation Response Behavior under Different Heat Flux Environments for Liquid Silicone Rubber Composites”

Correction to “Ablation Response Behavior under Different Heat Flux Environments for Liquid Silicone Rubber Composites”

Effect of Vinyl-Terminated Silicone Oil Chain Length and Reinforcing Agent on the Properties of Silicone Rubber

To solve the problems of low strength and high viscosity of room temperature vulcanized liquid silicone rubber, a series of terminated vinyl silicone oil were designed and synthesized, and low viscosity and high strength silicone rubber were prepared by the mechanical reinforcing agent. the results show that the molecular structure of the vinyl-terminated silicone oil has a significant effect on the mechanical properties and viscosity of the silicone rubber, and the best performance is found when the content of vinyl-terminated silicone oil is 0.16%. The low viscosity and high strength silicone rubber prepared from it was reinforced by vinyl MQ resin and fumed silica, which had a significant effect on improving the performance. Its tensile strength increased to 5.03 MPa, elongation at break to 338.90%, and tear strength to 7.15 kN/m compared to conventional silicone rubber, while the hardness increased to 43°. The viscosity is 34.9 Pa•s. The compression modulus is 7.48 MPa.

Micro‐ and Nanotexturization of Liquid Silicone Rubber Surfaces by Injection Molding Using Hybrid Polymer Inlays

AbstractMicro‐ and nanotexturization of surfaces can give to the parts different advanced functionalities, such as superhydrophobicity, self‐cleaning, or antibacterial capabilities. These advanced properties in combination with the biocompatibility of Liquid Silicone Rubber are an interesting approach for obtaining high‐performance medical devices. The industrial production of surface textures in polymeric materials is through the replication technique, and the best option to attain a high production rate is injection molding. Moreover, its low viscosity during processing can provide an accurate replication capacity by the easy filling by capillarity of the microtextures. An innovative replicating technique for Liquid Silicone Rubber is presented by studying the replication of different shaped textures within a diameter range of between 2 and 50 µm. The copying process consists in the overmolding of a textured polymeric inlay obtained by nanoimprint lithography. At the end of the process, a textured part is obtained, while the imprinted film remains in the mold. The injection molding parameters are optimized to increase the replication accuracy, and their effect on texture replicability is analyzed and discussed. Finally, it is shown that the textured surfaces improve their wettability behavior, which is a necessary and important characteristic in the development of biomedical devices.

Transcutaneous Energy Transfer System for Cardiac-Assist Devices by Use of Inhomogeneous Biocompatible Core Material

A novel magnetic liquid silicone rubber composite core design applicable to wireless power transfer (WPT) for medical applications is presented. As a case study, the integration of WPT, including the proposed cores for the coils, with ventricular-assist devices (VADs) was investigated. This facilitates transferring 4.6 W of power wirelessly through the skin layer, which, thus, allows to disposal of the transcutaneous cable that connects the extracorporal batteries to the implanted blood pump. To enhance the coupling coefficient and consequently increase the WPT efficiency, the geometry of the cores was optimized. This reduces the temperature increase in the implant area. The field and thermal simulations were carried out with CST software. The electrical properties of the proposed design as obtained by simulation were validated by measurements. The simulation results perfectly coincide with the measured results. Finally, a new optimized design with additional high permeability core inserts is introduced to further increase the coupling coefficient. The results indicate that the coupling coefficient is increased from 0.31 to 0.52 with the optimized design compared to the case without cores. Moreover, by fixing the initial temperature as 37 °C, the maximum temperature is decreased from 38.19 °C to 37.27 °C while keeping the transferred power fixed. Its high magnetic conductivity, biocompatibility, low-loss property, flexibility, and Qi-standard compatibility make the proposed WPT design an excellent candidate for the wireless powering of heart-assist devices, such as VADs.

A mechanism leakage model of metal-bipolar-plate PEMFC seal structures with stress relaxation effects

This work addresses the issues of long-term leakage rate prediction, which is crucial for durability study of proton exchange membrane fuel cells (PEMFCs). A theoretical model is presented for the leakage rate of compressive seal structures in PEMFCs, based on the combination of three numerical techniques, namely Lattice-Boltzmann method (LBM) simulations for rough wall interfacial gaps, a numerical 3D rough-surface generation technique, and Finite-Element-Analysis (FEA) for micro-contact mechanics of single asperity. The model clearly reveals the quantitative influence of various factors on the leakage rate without any empirical regression coefficients, and therefore can be easily integrated with structural mechanics and aging mechanism analyses. Long term sealing performance comparisons with three types of rubber material identify liquid silicone rubber to have the optimal durability. When influences of water environment are taken into account, an accelerated degradation of sealing performance can be observed after approximately 3000 h of operation. In addition, the effect of stress loss on leakage rate can be effectively suppressed by reducing surface roughness, reducing gasket thickness and increasing strain level. The proposed theoretical model provides an effective approach for the design of metal-bipolar-plate PEMFCs seal structures.

SHPB experimental method for ultra-soft materials in solution environment

Biological soft tissues usually perform physiological functions in solutions, and the mechanical properties of the bio-soft matters in air and solution may differ from that in solution, but the measurement technique of the dynamic mechanical properties of ultra-soft materials in solution is still an unexplored realm. In order to develop a feasible method to test ultra-soft materials in solution by the split Hopkinson pressure bar (SHPB) technique, this paper uses the liquid silicone rubber (LSR) whose property is seldom affected by water and the purified water as a testing material and a solution, respectively. The double-striker electromagnetic driving SHPB system, which has been designed for ultra-soft materials, is adopted to carry out dynamic test for LSR in solution, meanwhile the testing process was recorded by a high-speed digital camera. The tests include three cases named as: Specimen in air, No specimen in water, Specimen in water. Specimen in air was set as a reference; No specimen in water was used to learn the mechanism of load transferring of the water and to set the correction of the effect of the water; Specimen in water was the standard test in water whose results must be modified for the characterization of materials. The high-speed images show that the water may be regarded as a steady flow until the constant strain rate loading ended. Based on such phenomenon and the unchanged volume of specimen and by simplifying the description of the flow field, a data correcting method was set up on the basis of the deduction of both the specimen deformation and fluid flow field to modify the experimental result tested in water. According to the comparison, experimental result in air shows good agreement with corrected result in solution, indicating the feasibility of our recommended method for the SHPB tests in solution. In this way, our research is of great significance in laying an important foundation in the new academic field of the dynamic mechanical behaviors of bio-soft matters in solution.