Silica-filled Silicone Rubber Research Articles

Modulation of Mechanical Properties of Silica-Filled Silicone Rubber by Cross-Linked Network Structure.

Associating molecular structure and mechanical properties is important for silicone rubber design. Although silicone rubbers are widely used due to their odourless, non-toxic, and high- and low-temperature resistance advantages, their application and development are still limited by their poor mechanical properties. The mechanical properties of silicone rubbers can be regulated by designing the cross-link density and cross-linking structure, and altering the molar contents of vinyl in the side groups of methyl vinyl silicone rubber (MVQ) leads to different cross-linking structures and cross-linking densities in the vulcanized rubber. Therefore, this study investigated the differences in molecular parameters and molecular chain structures of unprocessed MVQ rubbers with different vinyl contents. The results showed that MVQ rubbers with high vinyl contents were branched polymers, better facilitating the cross-linking reaction than MVQ rubbers with low vinyl contents. In addition, silicone rubbers with different vinyl contents were co-cross-linked to introduce an inhomogeneous cross-linked network in the silicone rubber to improve its mechanical properties. The cross-linked network properties were analysed by the Flory-Rehner model and Mooney-Rivlin plots, and it was found that the long chains in the sparsely cross-linked domains of the network favoured high elongation at break and the short chains in the densely cross-linked domains contributed to high modulus, which could satisfy the functions of reinforcing and toughening the rubber materials at the same time. It was also found by analysing the filler network and aggregate morphology that the inhomogeneous cross-linked network led to an improvement in the dispersion of silica in the rubber and a significant improvement in the mechanical properties of silicone rubber.

Evaluation of the abilities of atomic oxygen/ultraviolet resistance for the filled silicone rubber coating: Structures, properties, and aging mechanism

A series of experimental techniques were used to study the erosion-damage effect of a silica-filled silicone rubber coating exposed to simulated space environment (atomic oxygen and ultraviolet irradiation). The elastomer is known to be chemically and thermally stable, but there are insufficient data on its durability in the space environment. In the present work, atomic oxygen doses up to 3.8 × 1021 atoms·cm−2 and ultraviolet irradiation doses up to 360 equivalent sun-hours (ESH) were applied from Space Environment Simulation Measurement System, respectively. Since the aging and failure processes of the filled silicone rubber relate to different aspects of chemical structure and physics form, therefore, this study explored the use of chemical-mechanics methods to comprehensively evaluate the erosion-damage effect of materials. Morphology and chemical changes due to radiation were investigated by scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS), and nanoindentation test probed surface mechanical properties changed with exposure dose. Results demonstrated that, with increased exposure time and irradiation-induced free radicals，the aggregation state and molecular chain structure of the silica-filled silicone rubber have changed, which caused an increase in surface hardness and modulus. Taken together, the results indicated that oxidative cross-linking was the dominant degradation mechanism under the irradiation dose in this paper.

How the silica determines properties of filled silicone rubber by the formation of filler networking and bound rubber

Aiming to unveil the correlation among formula, hierarchical structure, and performance of silica filled silicone rubber foams (SRFs), mechanical and structural investigations are performed on three selected samples. Precipitation (P) and fumed (F) silica filled SRFs with diameters of primary particles around 20 nm (PA), 16 nm (FB), and 14 nm (FC) are used, namely SRPA, SRFB, and SRFC. The hierarchical structures are obtained by contrast variation-small-angle neutron scattering (CV-SANS) with an established three-level unified Guinier-exponential/power-law approach. The sizes of both aggregates (agg) and agglomerates (agl) levels have the correlation of SRPA > SRFB > SRFC. Mass-fractals df, agg of SRPA, SRFB, and SRFC are 2.16, 2.16, and 2.54, while df, agl of the set are 2.90, 2.52, and 2.54, respectively. CV-SANS also indicates that the static thickness of bound rubber (TSBR) of SRPA, SRFB, and SRFC decreases from 10.3 nm to 7.5 nm, and then to 6.5 nm. Interestingly, low-field 1H nuclear magnetic resonance (NMR) indicates that the dynamics thickness of bound rubber (TNBR) of the set increases from 3.33 nm to 4.35 nm, and then to 4.41 nm. The discrepancy between TSBR and TNBR is ascribed to the interface interaction. Mechanically, three samples present different behaviors, while SRPA and SRFB have similar extension modules and compressive relaxations. According to a modified constitutive model fitting, and with combining the structural information, the roles of filler-filler and filler-rubber interactions on the reinforcement of SRFs are discussed.

Green Thermoplastic Vulcanizates Based on Silicone Rubber and Poly(butylene succinate) via In Situ Interfacial Compatibilization.

Presenting a combination of sustainability and environmental friendliness, a new class of green and non-petroleum-based thermoplastic vulcanizates (TPVs) was successfully developed from silica-filled silicone rubber (FSR) and poly(butylene succinate) (PBS) via dynamic vulcanization. The phase morphology, interfacial compatibilization, and microstructural properties of FSR/PBS TPVs were investigated. Notably, a large number of FSR microparticles were observed and were dispersed in the continuous PBS phase, indicating complete phase inversion during the dynamic vulcanization. The fine phase morphology of FSR/PBS TPVs was achieved by a fine phase morphology of the SR/PBS premix, the good interfacial compatibility between the PBS phase and the cross-linked FSR phase, and complete phase inversion. The as-prepared TPVs possessed high tensile strength, good elastic behavior, easy processability, and reprocessability. These novel non-petroleum-based TPVs have potential applications in packagings, biomedical devices, and three-dimensional (3D) printing materials.

Effect of fluorescence labeling on mechanical properties of silica filled silicone rubber

The Silica filler has been successfully fluorescently labeled by bifunctional silane coupling agent and europium nitrate through chemical grafting method. The thermal weight loss of the labeled silica at 200 °C was 1.75%, which was lower than 3.57% before labeling. This indicated that the labeled material had good thermal stability below 200 °C, and the water content of the labeled filler decreased due to its reduced hydrophilicity on the surface of the filler. Polysiloxane was filled with labeled or unlabeled silica which had different filler loading levels (from 10 phr to 60 phr) by diluted mixing on the purpose of investigating effect of fluorescent labeling on the mechanical property of rubber composite. According to the results of laser confocal microscope 3D imaging and software calculation, the network connection rate increased from 23% to 60% with the increase of filling amount. When the filling contents are more than 40%, the mechanical properties of unlabeled silica filled silicone rubber decrease slightly, while the mechanical properties of labeled silica filled silicone rubber almost remain unchanged, which is related to the increase interaction of filler and matrix through organic functionalization on the surface of the latter. However, the influence trend of fillers on mechanical properties of the two systems is basically the same. Fluorescent labeled silica is expected to be an effective tool for the correlation between microstructure and mechanical properties.

The aging properties and phase morphology of silica filled silicone rubber/butadiene rubber composites

Large amounts of antioxidants are used in unsaturated rubber composites, such as butadiene rubber (BR), which would inevitably cause surface discoloration. In this study, silicone rubber (VMQ) was blended with BR for improving its anti-aging properties. It was found that VMQ/BR exhibits better thermal oxidative aging and ozone aging resistance than BR, especially for 20/80 VMQ/BR. Atomic force microscopy (AFM) and transmission electron microscopy (TEM) were applied to characterize the phase morphology of VMQ–silica master-batch/BR, indicating that VMQ phases present island-dispersed domains of circular and elliptical shapes in the BR matrix, and silica particles mainly exist at the interface between BR and VMQ phases. These are two decisive factors for the improved aging properties of VMQ/BR.

Performance evaluation of alumina trihydrate and silica-filled silicone rubber composites for outdoor high-voltage insulations

In recent years, silicone rubber-based composites have been widely investigated for outdoor applications due to their promising insulating properties. However, mechanical, thermal, and tracking properties of pure silicone rubber are very poor, which restrains its application for long-term performance. In this research work, the influence of microsized alumina trihydrate (ATH) and micro/nanosized silica (SiO$_{2})$ fillers on mechanical, thermal, and electrical properties of room temperature vulcanized silicone rubber (RTV-SiR) has been studied. SiR-blends with varying amounts of ATH and SiO$_{2}$ particles were prepared by blending in a two-roll mixing mill, compression molding, and postcuring processes in sequence. In order to evaluate relative tracking and erosion resistance of SiR-blends, an inclined plane test (IPT) was conducted in accordance with the ASTM D2303 standard procedure. Surface temperature distribution was recorded using a Fluke-Ti25 infrared camera during IPT experiments. Thermogravimetric analysis was carried out to analyze the thermal stability of ATH and silica-filled silicone rubber samples. Tensile strength, percent elongation at break, hardness, erosion, tracking resistance, and thermal properties were also investigated and discussed. Results showed that the mechanical, thermal, tracking, and erosion performances of SiR-blends are improved by the incorporation of ATH and silica particles, which is governed by filler type, size, and wt. % in the polymer matrix.

Correlation between mechanical properties and microscopic structures of an optimized silica fraction in silicone rubber

The mechanical properties and the hierarchical filler structures were characterized on a series silica-filled silicone rubber with a filler fraction (ΦSi) varied from 0.05 phr to 80 phr (parts per hundred rubber). Uniaxial elongation measurement suggests that there is a percolation threshold between 10 and 30 phr. Moreover, an optimum ΦSi range from 40 phr to 50 phr is found, in which the best mechanical performances of reinforcement are shown. The microscopic structures were crosschecked by small-angle neutron scattering (SANS) and scanning electron microscopy (SEM). The effects of the ΦSi and the fabrication process on the morphology of samples are unveiled. The correlation length among aggregates extracted from SANS data monotonically decrease from 237.0 nm to 136.5 nm with increasing the ΦSi from 30 phr to 80 phr. The average radius of gyration of aggregates 〈Rg,agg〉 fitted with the Beaucage model monotonically decrease from 49.2 nm to 37.5 nm with increasing ΦSi from 10 phr to 80 phr. Providing a 10 nm thickness bound rubber as bridge, samples with optimum ΦSi yield a morphology that the radii of aggregates and the gap filled with polymer matrix in between are equivalent as both around 60 nm.

Parameterization of silica-filled silicone rubber morphology: A contrast variation SANS and TEM study

The hierarchical structures of silica-filled silicone rubber have been investigated by using contrast variation small-angle neutron scattering (CV-SANS) and transmission electron microscopy (TEM). A series of scattering intensities of swollen samples were measured with varying ratio of hydrogenated toluene/deuterated toluene solvent. By using CV-SANS method, the partial scattering functions, self-terms of silica SSS(q) and silicone rubber SRR(q), as well as cross-term between silica and silicone rubber SRS(q), are successfully extracted. The fine parameterization of morphology is characterized and elucidated by adopting unified Guinier-exponential/power-law equations, the fitted results indicate: (i) the volume fraction of silicone rubber in bound rubber and matrix regions, are respectively 0.80 and 0.31. Such spatial inhomogeneous of swelling ratio suggests that there is existence of the immobilized layer; (ii) the average aggregation size is ca. 143 nm, and there are about 24–97 silica particles in each aggregation; (iii) the thickness of the bound rubber is estimated as in the range of ca. 7.2–8.5 nm. A mimic collection and observation of aggregations and bound rubber with TEM confirm the CV-SANS results. Current work provided a scenario to evaluate the reinforcement effect of the silica on the silicone rubber even before any mechanical testing.

Performance of silicone rubber composites with SiO2 micro/nano-filler under AC corona discharge

Outdoor insulators are often subject to corona discharges and the problem is becoming more prevalent with the increasing use of higher transmission voltage levels. For polymeric insulators, exposure to such discharges can alter the chemical structure of basic polymer and degrade surface properties. This paper investigates the effect of micro and/or nano fillers in silicone rubber composites in suppressing such damage. Four different types of samples are fabricated: pristine silicone rubber (PR), 30wt% micron-sized silica/silicone rubber (MC), 27.5wt% micron + 2.5wt% nano silica/silicone rubber (NMC), and 5wt% nano silica/silicone rubber (NC) composites. Samples are exposed to AC corona using a needle to ground-plane electrode setup. Experimental results are analyzed based on five different measurement methods: phase-resolved partial discharge (PD), hydrophobicity loss-recovery, Scanning Electron Microscopy (SEM), Surface roughness and Fourier Transform Infrared Spectroscopy (FTIR). Results indicate that NC shows a strong resistance to partial discharges and hydrophobicity loss. In the area below the needle tip, higher hydrophobicity loss and higher recovery are observed as compared to the vicinity region. Variations in surface roughness, appearance of crackles, voids, pits, surface splitting into blocky structures and damages to chemical structure of silicone rubber are appreciably retarded in NC as compared to PR, MC and NMC. Based on NMC results, it is found that addition of nano-sized silica can be an attractive approach to improve the corona resistance of micron-sized silica filled silicone rubber.

The effect of bound rubber on vulcanization kinetics in silica filled silicone rubber

Different filler networks mediated by bound rubber show obviously different effects on vulcanization kinetics.

Preparation and properties of hollow glass bead filled silicone rubber foams with low thermal conductivity

Silicone rubber foams filled with various content and different particle size of hollow glass bead (HGB) were prepared by compression molding. It was revealed that compared with silica filled silicone rubber foams, HGB filled materials achieved higher foaming extent, lower thermal conductivity, and lower hardness, which can be significant for thermal insulation materials. For HGB filled materials, the morphology indicated the average cell size decreased with higher HGB content and larger particle size of HGB. The density, thermal conductivity, hardness and tensile strength increased with higher HGB content and larger particle size of HGB.

Competition Between Chemical Network Formation and Physical Adsorption and Desorption Processes in a Silica-Filled Silicone Rubber: Calorimetry versus Refractometry Studies

Calorimetry and high-performance refractometry are applied to study the network formation and accompanying morphological changes in reactive polydimethylsiloxane systems filled with silica nanoparticles. Both methods give insight into the structure formation during the polymerization of silica-filled silicone nanocomposites in an impressively complementary way. It will be shown that the specific heat flow as determined by calorimetry does not allow for estimating the chemical conversion whereas the refractive index does, which results from the different perspectives of both techniques on the relevant polymerization, adsorption, and desorption processes occurring in the nanocomposites. Finally, the interphase formation in the polymeric matrix in the vicinity of the nanoparticles and the network formation within the silicone rubber are demonstrated to be strongly correlated processes.

Concentration Effects of Organosilane (TESPD) on Mechanical Properties of Silica Filled Silicone Rubber/Natural Rubber Compounds

Abstract The concentration effects of the silane coupling agent bis (triethoxysilylpropyl) disulfide (TESPD) on mechanical properties of silica-filled silicone rubber (SR)/natural rubber (NR) compound are investigated upon torque maximum value, tensile strength, tensile modulus and hardness. The compounds are cured with a conventional vulcanization system. The concentration range of the TESPD used is 1 to 17 phr. The optimum concentration of the TESPD is found to be 13 phr in the SR/NR compound. The degree of dispersion of silica in the rubber matrix is also examined by scanning electron microscopy (SEM).

Vinyl Functional Group Effects on Mechanical and Thermal Properties of Silica-Filled Silicone Rubber/Natural Rubber Blends

The silane (TESPD) treated silica-filled silicone rubber (SR)/natural rubber (NR) blends are studied with regard to their mechanical, thermal, and optical properties. Two grades of SR, with and without vinyl group are used and the results are compared. The effects of a vinyl functional group in SR with respect to vibration isolation property, and mechanical properties are investigated to find maximum torque, tensile strength, tensile modulus, elongation at break, hardness and tan δ. The compounds are cured with sulfur, peroxide and peroxide/sulfur combination. The results show that the presence of the vinyl group in SR helps improve the mechanical and thermal properties of the SR/NR blend. Scanning electron microscopy (SEM) photographs show that the silica particles are well dispersed in the rubber matrix. Thermal properties including thermogravimetric analysis (TGA), differential scanning calorimetry (DSC) and heat resistance are also studied. It shows the presence of the vinyl group in the SR/NR blend curing with the peroxide/sulfur combination improves both mechanical properties and the vibration properties.

Ultrasonic Devulcanization of Precipitated Silica-Filled Silicone Rubber

Abstract This investigation involves the recycling of precipitated silica-filled silicone rubber using a continuous ultrasonic reactor. The processing conditions utilized in devulcanization of the filled systems were similar to those in our previous work on unfilled systems. Significant differences were observed in devulcanization of unfilled and filled systems. A decrease in gel fraction and crosslink density was sufficient for devulcanized silica-filled silicone rubber to be reprocessed and revulcanized. The cure behavior of silicone rubber and the mechanical properties of virgin, and revulcanized rubber were measured. Results showed that unfilled silicone rubbers gave no change in vulcanizate mechanical properties after revulcanization. However, in the filled rubbers there was a decrease in the mechanical properties of revulcanizates. In order to achieve an improvement in the properties, devulcanized filled rubbers were blended with virgin filled rubber in various proportions. The obtained results indicated a considerable enhancement of the performance characteristics of these blend vulcanizates.

Effect of Precipitated Silica Physical Properties on Silicone Rubber Performance

Abstract Nitrogen BET surface area is a highly accurate predictor of the physical performance of silica-filled and silica and carbon black-filled, cured organic rubber compounds. The effectiveness of predicting performance of a high-temperature-vulcanized, high-consistency silicone rubber, based on precipitated silica physical properties, is presented. Correlation and regression analyses were performed for twenty-five silica physical properties and fourteen silicone rubber performance characteristics. Silica properties studied included measurements of surface area, structure, impurities, particle size, pH and adsorbed moisture. Silicone rubber performance included measures of processing, curing, reinforcement, resiliency, stress whitening, color and clarity. The present study confirms that no single silica physical property can predict silicone rubber performance. A combination of surface area, oil absorption, particle size, pH, adsorbed moisture, residual sodium and/or residual salt can qualitatively predict the physical performance of high-temperature-vulcanized, high consistency, precipitated silica-filled silicone rubber.

Ammonia-Induced Tensile Set and Swelling in Silica-Filled Silicone Rubber

Abstract A new experimental procedure has been proposed for estimating the level of surface interaction activity of silica fillers in silicone rubber vulcanizates. The method is based on a simultaneous effect of the constant tensile strain and ammonia. The sample of silica-filled silicone rubber vulcanizate is deformed in an ammonia atmosphere for 72 h. The ammonia cleaves the specific silica—PDMS hydrogen attachments. The bonds are formed in new positions in the deformed polysiloxane network between silica surface free silanols and PDMS after evaporation of ammonia. The resulting permanent deformation (tensile set) provides a measure of silica surface reactivity (interaction activity) with respect to a given silicone elastomer. It has been proven that the ammonia-induced tensile set is governed by the same mechanism as the increase of toluene swelling under an ammonia atmosphere. The results indicated that water plays an important catalytic role in the ammonia-induced cleavage of silica-PDMS attachments. It is recommended to use an NH4OH atmosphere to obtain the maximum cleavage when estimating the contribution of polymer-filler bonding (hydrogen or physical) to the total crosslink density in silica-filled silicone rubber.

Semiempirical Relationships between Properties and Loading in Filled Elastomers

Abstract The dynamic storage modulus of uncured silica-filled silicone rubber increases sharply when a critical filler loading is reached. A similar behavior has been observed for the conductivity in the same compounds. The experimental data fit a percolation law fairly well, and microscopy confirms the formation of a filler network according to the percolation model. This behavior depends on a polymer-filler interaction level, suggesting a filler network consisting of filler particles bridged together by adsorbed polymer. It has been found that the percolation threshold coincides with the optimum value of elongation at break of the corresponding vulcanizates. The percolation behavior and the relations between properties of filled rubbers described here were observed in various filled rubbers of quite different compositions. This seems to suggest the general validity of the presented relationships in all elastomers reinforced by particulate fillers.

NH3‐modified swelling of silica‐filled silicone rubber

AbstractThe NH3‐modified swelling method was used for fume silica‐filled silicone rubber compounds and vulcanizates. The method provides an estimation of the polymer–filler interaction in the system. Bound rubber reduction due to the modified swelling is shown to be dependent on the silica surface silanol content in the compound. The swelling behavior of silica‐filled vulcanizates was also studied in an NH3 atmosphere. The fraction of polymer–filler attachments in the vulcanizates was found not to depend on the particle size of silica and its surface modification, depending only on the total polymer–filler contact area in the system. No relation was found between bound rubber content in a compound and swelling of a filled vulcanizate of the same composition. The strong rubber‐to‐filler adherence found in toluene changed to nonadherence under NH3‐modified swelling conditions. The NH3‐induced cleavage of silica–silicone rubber attachments was found to be nearly completely reversible. In the silica‐filled silicone rubber vulcanizates, the fraction of polymer–filler attachments due to polymer–filler interaction represents a substantial contribution to the total number of network chains.