Silicone Rubber Foam Research Articles

Mechanical Characteristics of Multi-Level 3D-Printed Silicone Foams.

Three-dimensional-printed silicone rubber foams, with their designable and highly ordered pore structures, have shown exceptional potential for engineering applications, particularly in areas requiring energy absorption and cushioning. However, optimizing the mechanical properties of these foams through structural design remains a significant challenge. This study addresses this challenge by formulating the research question: How do different 3D-printed topologies and printing parameters affect the mechanical properties of silicone rubber foams, and how can we design a novel topological structure? To answer this, we explored the mechanical behavior of two common structures-simple cubic (SC) and face-centered tetragonal (FCT)-by varying printing parameters such as filament spacing, filament diameter, and layer height. Furthermore, we proposed a novel two-level 3D-printed structure, combining SC and FCT configurations to enhance performance. The results demonstrated that the two-level SC-SC structure exhibited a specific energy absorption of 8.2 to 21.0 times greater than the SC structure and 2.3 to 7.2 times greater than the FCT structure. In conclusion, this study provides new insights into the design of 3D-printed silicone rubber foams, offering a promising approach to developing advanced cushioning materials with superior energy absorption capabilities.

Fabrication of CoFe‐layered double hydroxide for flame retardant and smoke suppression of silicone rubber foam

AbstractIn order to improve the flame retardancy and smoke suppression of silicone rubber foam (SRF), the CoFe‐LDH was fabricated by coprecipitation method and used as flame‐retardant ingredient in SRF. The influence of CoFe‐LDH on the flame retardancy, combustion behavior, and thermal stability of SRF was investigated. The results demonstrate that SRF containing only 1.0 phr CoFe‐LDH can pass V‐0 rating in the vertical combustion test, with a limiting oxygen index of 28.6%. The total heat release and total smoke production are 22.2% and 61.1% lower than that of SRF, respectively. Moreover, CoFe‐LDH can improve the thermal stability of SRF in the high temperature range, and the char residue at 900°C of SRF/LDH composite is obviously higher than that of pure SRF. In addition, to further understand the flame‐retardant mechanism of CoFe‐LDH, the micromorphology of the char residue and the gaseous products during heating for SRF and SRF/LDH composite were analyzed. The results indicate that CoFe‐LDH exhibits excellent flame‐retardant effects in both condensed and gaseous phases for its functions of catalysis, carbonization, and nonflammable gas release. This study provides a more efficient and convenient strategy for the flame retardant and smoke suppression research of SRF.

Facile preparation of conductive silicone rubber composite foams with tunable cell morphologies and absorption-dominant characteristics

Flexible silicone rubber (SR) foams with tunable cellular morphologies were fabricated via supercritical CO2 foaming. Conductive carbon black (CB) modified at a low cost and with high complex viscosity was preferentially selected. The effects of the pore size and void fraction on the electrical conductivity and dielectric permittivity of the SR/CB composite foams were carefully investigated. The pore size of the foams affected the specific shielding effectiveness (SSE) and absorption coefficient (A), whereas the variation in the void fraction did not generate evident changes. In comparison with its solid counterpart, a foam with a pore diameter of 59.9 μm showed a 50 % decrease in density, 31.6 % increase in absorptivity, and 95.7 % increase in SSE, demonstrating a considerable electromagnetic interference shielding effectiveness (EMI SE). Decreases in the pore size and void fraction of the foams improved compression modulus and strength. In addition, sample preparation process was simplified, making industrial production easier.

Large-scale and facile fabrication of phenyl-containing silicone foam materials with lightweight, wide-temperature flexibility and tunable pore structure for exceptional thermal insulation

Silicone foam materials with unique inorganic/organic molecular networks and porous structures are widely used in many emerging fields from aerospace to new energy fields. However, current silicone foam materials still show some limitations, such as the relatively high-density values of >200 mg cm−3, complex fabricating process and difficulty in tailoring pore structure. Herein, we report a large-scale and facile fabricating strategy to prepare phenyl-containing silicone foam materials (PhSiRF) with ultra-lightweight feature, tunable pore structure, and excellent wide-temperature mechanical flexibility. Interestingly, the presence of phenyl groups onto the Si–O–Si backbone tailors the chemical foaming rate probably due to the steric hindrance effect, thus producing tunable pore size distributions in the range of 180–500 μm. Typically, the PhSiRF with 50 % phenyl groups not only shows a very low density of ∼100 mg cm−3, superior to previous silicone foams and composites, but also exhibits wide-temperature flexibility (stable compressive strain of 80 % from −90 to 210 °C) and excellent thermal insulation performance, which outperforms those of conventional polymer foams including polyurethane, polyethylene and melamine foams. Based on the structure observation and theory analysis, the influence of different pore morphological structures on the thermal insulation performance is discussed and demonstrated. Clearly, this work provides a new yet simple method for developing high-performance silicone rubber foam materials for promising thermal insulation applications.

Flame retardancy and smoke suppression effect of zinc borate-graphite intercalation compound/Fe2O3 on silicone rubber foams

To enhance the flame retardancy and smoke suppression of silicone rubber foams (SRFs), a novel flame retardant, zinc borate-graphite intercalation compound (ZB-GIC), was synthesized and used in conjunction with Fe2O3. The results showed that ZB-GIC markedly improved the flame retardancy and smoke suppression of SRFs, and Fe2O3 significantly improved the smoke suppression of SRFs. ZB-GIC and Fe2O3 had an excellent synergistic effect on flame retardant and smoke suppression. 5 wt% ZB-GIC synergistically with 5 wt% Fe2O3 resulted in SRFs passing the UL-94 V-0 rating, whose limiting oxygen index (LOI) value reached up to 43.7%. Its peak heat release rate (PHRR), total heat release (THR), peak smoke production rate (PSPR), and total smoke production (TSP) decreased by 67.42%, 89.55%, 76.86%, and 91.67% compared to pure SRF. The ZB-GIC produced incombustible gas and a large amount of H2O when heated, which reduced the intensity of gas-phase combustion. The ZB filled the interlayer gaps formed after the expansion of GIC to enhance the strength of the char layer, and Fe2O3 further strengthened the char layer. Our work has greatly reduced the fire hazards of SRFs and provided a promising flame retardant for the large-scale production of high-performance SRFs.

High Consistency Silicone Rubber Foams.

Silicone elastomers are high-performance plastics. In the extrusion process, only high-consistency silicone rubbers were used. In order to reduce the cost and weight, silicone rubbers can be foamed during processing. In this study, high-consistency silicone rubber is processed with different physical and chemical blowing agents. The resulting reaction kinetics, as well as the mechanical and morphological properties, had been investigated and compared with each other. This showed that the chemical blowing agent significantly influenced the crosslinking reaction compared to the microspheres and the water/silica mixture tested, but it also achieved the lowest density compared to the physical blowing agents. When evaluating the foam morphology, it became clear that the largest number of pores was achieved with the microspheres and the largest pores when using the water/silica mixture. Furthermore, it has been shown that the different mechanisms of action of the blowing agents have a major influence on the mechanical properties, such as the micro shore hardness and the foam morphology.

A facile and efficient strategy for fabrication of silicone rubber foam based on in situ precuring‐supercritical CO2 foaming technology

AbstractIn this work, a facile in situ precuring and supercritical CO2 (sc‐CO2) foaming technology is proposed to fabricate silicone rubber foam (SRF) with a tunable cell structure. Uncured silicone rubber is put into the autoclave to saturate sc‐CO2 directly, which not only shorts the saturation time but also accelerates the curing reaction in the precuring process. In the foaming process, cell nucleation is triggered by rapid release of pressure. The cell structure of SRF samples with cell size range of 9.45 to 128.09 μm, cell density range of 3.5 × 105 cells/cm3 to 5.7 × 108 cells/cm3, and porosity range of 28.35% to 77.28% are successfully fabricated by controlling of precuring time, saturation pressure and foaming temperature. To guarantee the dimensional stability and optimize surface quality of SRF samples, in situ post‐curing process is implemented within the foaming mold. With the increase of cell size from 9.91 to 128.09 μm, the compressive stress at strain of 75% decreases from 2.19 to 0.61 MPa, while the energy loss coefficient reduces from 35.4% to 16.9%, exhibiting excellent compression recovery steady. The new technique integrates the precuring, foaming process and post‐curing into an autoclave to simplify the SRF procedure.

3D interconnected graphene nanoplatelets and nickel ferrite based silicone rubber foams for effective electromagnetic interference shielding and thermal insulation performance

In the present study, porous composite foams made of silicon rubber (SR), graphene nanoplatelets (GnP), and nickel ferrite (NiFe2O4) nanoparticles were developed using the salt template approach. SR act as a 3D porous network, the conductive channel provided by graphene nanoplatelets and the magnetic and dielectric loss provided by the NiFe2O4 and the composite foam as a whole showed absorption-dominated EMI shielding due to synergism between GnP and NiFe2O4. Without NiFe2O4, the total EMI SE is 23 dB, while different NiFe2O4 weight percentages (0%, 3%, 6%, & 9%) increase the SET up to 36.7 dB. The thermal conductivity of this prepared sample is found to be 0.0313 Wm−1K−1 for 9 wt% filler content of NiFe2O4. It shows a moderately low value as compared to existing polymer composites. The resulting SR/GnP@NiFe2O4 composite foams, which are lightweight, strong, and multifunctional, have straightforward synthesis procedures and have a myriad of possible uses for EMI shielding systems and thermal insulations in the disciplines of aerospace, military engineering, electrical engineering and electronics.

Effect of polydiborosiloxane on the mechanical properties of silicone rubber foam with dual network structure

AbstractA silicone rubber foam is prepared by introducing polydiborosiloxane (PDBS) into silicone rubber, and using expandable microspheres as blowing agents. The influences of the dual network structure with dynamic cross‐linking network on the silicone rubber foam is characterized by the cell size, mechanical properties and impact resistance. The cell size increases from 77 to 93 μm due to the introduction of a dynamic cross‐linking network which reduces foaming resistance. The elongation at break elevates from 277% to 574%, and the yield strength gradually decreases. The dynamic cross‐linking network also enhances the internal damping characteristic and energy absorption capacity of the foam. The impact resistance of silicone rubber foam is improved because of the synergistic effect of the dual network structure and the bubble structure. The research improves the performance of silicone rubber foam.

Porous structures design in liquid silicone rubber foams through CO2 controlled molecular crosslinking

During CO2 foaming, molecular crosslinking plays an important role on cell structure of rubber foam. In this work, effects of CO2 on crosslinking reaction of liquid silicone rubber (LSR) were investigated according to viscoelasticity and reaction activation energy. It was found that CO2 facilitates formation of local primary crosslinking structure in LSR, while inhibiting creation of crosslinking network. When saturation pressure was lower than 13 MPa, LSR foam can obtain uniform cells only under a narrow saturation time. After increasing CO2 pressure higher than 13 MPa, cell structure was less affected by saturation time and time window for preparing uniform cell structure was extended. LSR foam was prepared with minimum cell size of 22.1 µm as well as maximum expansion ratio of 4.9. This work demonstrates CO2 controlled molecular crosslinking is key to porous design of LSR, which exhibits good theoretical exploration and application value in preparation of LSR foam.

The Effects of a Crosslinking Agent on the Microrheological Properties and Cellular Structure of Silicone Rubber Foam Prepared via a Green Process.

Chemical foaming technology is widely used in the preparation of silicone rubber foam and is attributable to its one-step molding capability and eco-friendly production processes. The microrheological properties of silicone rubber play a pivotal role during the foaming process. In this study, Rheolaser Lab (Formulaction, Toulouse, France) was used to conduct in situ examinations for the influence of a crosslinking agent on the microrheological properties of silicone rubber foam for the first time. This study monitors the entire reaction process of silicone rubber foam from liquid to solid, as well as the matching of crosslinking and foaming reactions. Various parameters, including solid-liquid balance, elasticity index, and macroscopic viscosity index, are measured to analyze the microrheological properties of silicone rubber foam. The results show that the silicone rubber foam exhibits good microrheological properties, thereby demonstrating excellent performance at a crosslinking agent content of 2%. Through adjusting the experimental conditions, a sustainable and efficient approach was proposed for better cellular structure control in the industrial preparation of silicone rubber foam.

Molecular dynamics simulation and experimental verification of the effects of vinyl silicone oil viscosity on the mechanical properties of silicone rubber foam.

The molecular motion trajectories of silicone rubber foam (SRF) at various vinyl silicone oil viscosities were studied via molecular dynamics (MD) simulation from the perspective of all atomic molecules. The influence of different viscosities of vinyl silicone oil on interaction, compatibility, and aggregation degree of molecules was determined based on the mean square displacement, diffusion coefficient, binding energy, solubility parameter, radial distribution function, and radius of gyration. The mechanical properties of the SRF were also experimentally verified. Results revealed that as the viscosity of vinyl silicone oil increased, the mean square displacement, fractional free volume, diffusion coefficient, and solubility parameter of the system decreased, whereas its larger radius of gyration increased. Moreover, the radial distribution function showed a weaker relative interaction between molecular chains. The calculated binding energy demonstrated that the system had better compatibility at a viscosity of 0.45 Pa s. This study provided a deeper insight into the relation between the viscosity of vinyl silicone oil and mechanical properties of the SRF. As the viscosity of vinyl silicone oil increased, the changing trend in MD simulation results of elastic modulus, shear modulus, bulk modulus, and Poisson's ratio was consistent with the experimental results. The MD simulations can promote theoretical predictions and scientific basis for the design of the SRF with desired performances.

Facile fabrication of low-content surface-assembled MXene in silicone rubber foam materials with lightweight, wide-temperature mechanical flexibility, improved flame resistance and exceptional smoke suppression

Silicone rubber foam (SiRF) is a promising porous material that is widely applied in various fields. However, its intrinsic flammability nature remains a significant challenge. One of the simplest and the most economical solutions is to incorporate high content flame-retardant fillers. Unfortunately, such approach is limited by several drawbacks, including poor filler dispersion, difficulty in processing and negative effect on mechanical flexibility. Herein, we report a green and facile strategy for surface-assembling ultralow content of MXene sheets bonded onto the pore surface of SiRF via the effective foaming reaction process. The as-prepared MSiRF-0.2 composite at 0.2 wt% MXene exhibited lightweight characteristic, wide-temperature mechanical flexibility, UL94-V0 grade, limited oxygen index of > 27 %, and exceptional smoke suppression (reduction by 88.24 %). Furthermore, flame-retardant mechanism was discussed based on the char structure observation. This study provides a novel approach for fabricating mechanically flexible and flame-retardant SiRF via in-situ reactive assembly of low content MXene sheets.

Preparation of phase-change microcapsules with illite as a filler and their applications in foaming materials

Abstract To solve the problem of the leakage of phase-change materials and expand the application of clay minerals in the field of phase-change materials, illite and paraffin wax were used for the first time as a new binary hybrid core material with silica as the wall material. Phase-change microcapsules were prepared using the sol–gel method and successfully applied in silicone rubber foams. Differential scanning calorimetry showed that the microcapsules had a melting temperature of 62.99°C and a latent heat capacity of 98.24 J g–1, indicating their good heat-storage capacity. Furthermore, the effectiveness of using illite as a filler for preventing paraffin leakage was demonstrated through 200 thermal cycling tests, which demonstrated a leakage rate of only 2.6% compared to 3.2% exhibited by microcapsules without illite.

Foaming behavior regulation of silicone rubber via evolution of crosslinked structure

The cellular structure and foam density of silicone rubber foam are known to be influenced by its rheological behavior, which could be adjusted by crosslinked density and crosslinked structure. However, the effect of the crosslinked structure evolution in methyl vinyl silicone rubber (MVQ) on the supercritical foaming behavior has never been investigated. In this research, the crosslinked density and T2 relaxation time of MVQ were charactered by low-field nuclear magnetic resonance (LF-NMR), which revealed the evolution of crosslinked structures and change of the crosslinked density under different vinyl group contents and pre-cure time in MVQ. In addition, a processing strategy which utilized conjunctively crosslinked structure and crosslinked density of MVQ was used to design the cellular structure and prepare low density foam. The resulting MVQ foam had an adjustable density range from 1.12 g/cm3 to 0.08 g/cm3 and different cellular structures. In this study, the effective control of foaming behavior was realized by manipulating the crosslinked density and crosslinked structure, and this approach would pave a new possibility to obtain desired cellular structures.

A study on flame retardancy and smoke suppression of diatomite/hexaphenoxycyclotriphosphazene in silicone rubber foam

A study on flame retardancy and smoke suppression of diatomite/hexaphenoxycyclotriphosphazene in silicone rubber foam

Flame retardancy and mechanical properties of silicone rubber foam composite reinforced with ZnNiAl layered double hydroxides

Flame retardancy and mechanical properties of silicone rubber foam composite reinforced with ZnNiAl layered double hydroxides

Self-Assembly of Multiwalled Carbon Nanotubes on a Silicone Rubber Foam Skeleton for Durable Piezoresistive Sensors.

Conductive nanomaterial/flexible polymer composite foams are of great interest in the field of flexible and wearable piezoresistive pressure sensors. However, the existing composite foam sensors are faced with stability issues from conductive nanomaterials, which tends to decrease their long-term durability. Herein, we developed a solvent evaporation-induced self-assembly strategy, which could significantly improve the stability of multiwalled carbon nanotubes (MWCNTs) on a silicone rubber foam skeleton. The process for self-assembly of MWCNTs was straightforward. Aqueous MWCNT dispersion droplets were first hierarchically enclosed in silicone rubber via water-in-oil (W/O) Pickering high internal phase emulsions (HIPEs). Then, the high pressure generated by fast evaporation of the solvent from the droplets could break the thinnest pore walls to form interconnected pores. As a result, very dense and firm MWCNT layers were self-assembled on the pore wall surface. Due to the excellent stability of MWCNTs and tetramodal interconnected porosity, our MWCNTs/silicone rubber composite foam showed the following "super" properties: low density of 0.26 g/mL, high porosity of 76%, and excellent mechanical strength (the maximum stress loss of 8.3% at 80% strain after 100 compression cycles). In addition, excellent piezoresistive performance, including superior discernibility for different amplitudes of compressive strain (up to 80%), rapid response time (150 ms), and high sensitivity (gauge factor of 1.44), was demonstrated for such foams, together with prominent durability (39,000 compression cycles at 60% strain in air) and excellent stability of resistance response in water and organic solvents (5000 compression cycles at 30% strain in water and ethanol). Regarding its application, a wearable piezoresistive sensor, which was assembled from the as-prepared conductive silicone rubber composite foam, could capture various movements from tiptoeing and finger bending to small deformations resulting from human pulse.

Effect of Al‐based metal‐organic frameworks/halloysite nanotubes on the flame retardancy of silicone rubber foam

AbstractTwo kinds of Al‐based metal‐organic frameworks (Al‐MOFs), MIL‐53(Al), and MIL‐96(Al), were synthesized by the solvothermal method and then characterized using SEM, XRD, and FTIR. The Al‐MOFs and halloysite nanotubes (HNTs) were blended and added to silicone rubber foams (SiFs). The mechanical properties and flame retardancy of the composites were tested and analyzed using universal tensile tester, limiting oxygen index (LOI), vertical combustion tester, cone calorimeter, TG, SEM, EDS, and FTIR. The results show that Al‐MOFs synthesized by the solvothermal method have high crystallinity and purity. Compared with pure SiFs, the LOI of SiFs with 2 wt% MIL‐53(Al) and 2 wt% HNTs increased to 29.4%, while total heat release and total smoke release decreased by 36.9% and 32.4%. The flame retardant mechanism of the composites is mainly reflected in the condensed phase. MIL‐53(Al) and HNTs can synergistically promote the formation of a dense carbon layer, blocking heat and oxygen from entering the SiFs matrix.

Key factors for regulation of cell morphology in supercritical CO2 direct rapid depressurization foaming silicone rubber process

Silicone rubber foam was prepared by direct rapid depressurization foaming process using supercritical CO2 as foaming agent. It was found that under high temperature and pressure, the dissolution of CO2 and the vulcanization reaction occurred simultaneously. The vulcanization degree range with good cell morphology was between 19.8% and 48.2%. When the saturation pressure exceeded 16 MPa, the cell morphology did not depend on foaming pressure. When the temperature was increased from 110 °C to 140 °C, the degree of vulcanization increased from 1.8% to 60.5%, and the cell size ranged from 13.9 µm to 646.4 µm, with an order of magnitude variation in cell density. This study identified the key control factors for supercritical CO2 relief foaming of silicone rubber and provided a fast and efficient preparation method, as well as theoretical guidance, for obtaining lightweight microporous silicone rubber foams.