Search

In this study, silicone rubber (SiR) with 0, 90, and 180 parts of aluminum hydroxide (Al(OH)3, ATH) contents prepared in the laboratory was treated in a certain concentration of NO2 for 0, 12, 24, and 36 h. Fourier transform-infrared spectroscopy (FT-IR), scanning electron microscopy (SEM), atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS), and thermogravimetry (TG) were used to study the changes in the surface structure and thermal stability of SiR, as well as the influence of Al(OH)3 on the properties of SiR. According to AFM, the root-mean-square roughness of ATH-90 SiR was 192 nm, which was 2.7 times of ATH-0 SiR. With the incorporation of ATH, the surface of SiR became more susceptible to corrosion by NO2. According to FT-IR and XPS, with the increase in aging time, the side chain Si-CH3 of polydimethylsiloxane (PDMS) was oxidized gradually and a few of nitroso -NO2 groups were formed. According to TG, the incorporation of ATH caused the maximum decomposition rate temperature of PDMS to advance from 458.65 °C to 449.37 and 449.26 °C, which shows that the thermal stability of SiR degraded by adding ATH. After NO2 aging, a new decomposition stage appeared between 75 and 220 °C (stage Ⅰ), and this decomposition trend was similar to aluminum nitrate, which was proven to reduce the thermal stability of PDMS. The effects of NO2 on the surface structure and thermal stability of different ATH contents of silicone rubber were preliminarily clarified by a variety of characterization methods, which provided ideas for the development of silicone rubber resistant to NO2 aging. [ABSTRACT FROM AUTHOR]

In this study, functional graphene oxide (f-GO) nanosheets were prepared to enhance the NO2 resistibility of room-temperature-vulcanized (RTV) silicone rubber. A nitrogen dioxide (NO2) accelerated aging experiment was designed to simulate the aging process of nitrogen oxide produced by corona discharge on a silicone rubber composite coating, and then electrochemical impedance spectroscopy (EIS) was used to test the process of conductive medium penetration into silicone rubber. After exposure to the same concentration (115 mg·L−1) of NO2 for 24 h, at an optimal filler content of 0.3 wt.%, the impedance modulus of the composite silicone rubber sample was 1.8 × 107 Ω·cm2, which is an order of magnitude higher than that of pure RTV. In addition, with an increase in filler content, the porosity of the coating decreases. When the content of the nanosheet increases to 0.3 wt.%; the porosity reaches a minimum value 0.97 × 10−4%, which is 1/4 of the porosity of the pure RTV coating, indicating that this composite silicone rubber sample has the best resistance to NO2 aging. [ABSTRACT FROM AUTHOR]

Silicone rubber is widely used as an insulating material. In this article, silicone rubber samples were prepared by varying the content of crosslinker (2,5-bis(tert-butyl-peroxy)-2,5-dimethylhexane, DBPMH), and the free volume holes in the samples were investigated by means of positron annihilation lifetime spectroscopy (PALS) measurement. The surface chemical structure, surface micromorphology and water diffusion of the samples after corona discharge treatment were studied by FTIR, SEM and EIS measurements, respectively. As the crosslinker weight ratio increased from 0.2 wt.% to 1.5 wt.%, the mean free volume hole size first decreased and then remained unchanged. However, the concentration of free volume holes did not vary as the crosslinker weight ratio increased. SEM morphologies show that surface cracks were produced on samples having high crosslinking levels after corona treatment. The water diffusion coefficient of samples after corona treatment increased from 3.13 × 10−10 cm2 s−1 to 17.68 × 10−10 cm2 s−1 in the initial immersion period, as the crosslinker weight ratio increased from 0.2 wt.% to 3.0 wt.%. The results indicated that deterioration of samples with high crosslinking levels were more serious and water repellency more easily lost. The corona resistance ability of low crosslinking level silicone rubber stems from internal low molecular weight molecules. [ABSTRACT FROM AUTHOR]

Silicone rubber is widely used in power grids, electronics and aerospace, because of its hydrophobicity, high flexibility, etc. However, it is susceptible to some environmental chemical factors, such as NO2 and ozone, which can induce chain scission, the generation of defects and the ingress of water. In this work, we systematically studied the microstructure variations and water diffusion behavior of high-temperature vulcanized (HTV) silicone rubber after NO2-induced oxidative damage. Microstructure evolution in silicone rubber was investigated by positron annihilation lifetime spectroscopy (PALS). The results show that the oxidation of NO2 significantly degrades organic matrices, which induces the decrease of crosslinking degree and the formation of defects compared to virgin samples. The water transportation was evaluated by electrochemical impedance spectroscopy (EIS), which showed that the diffusion coefficient, under the NO2 concentration of 28.75 mg L−1, is 106 times that of virgin sample with a water uptake of 31%. Particularly, when the concentration reaches the critical value around 17.25 mg L−1, the diffusion coefficient and water uptake increase sharply, and the calculated average porosity of samples also dramatically increases by 4 orders of magnitude. These indicate the formation of more nano- and micron holes, serving as a pointer for percolation of defects in silicone rubber bulk. The study of microstructure variations and water transportation can help us to understand the aging mechanism, design reasonable composite polymer materials and prevent the damage of chemical contact. [ABSTRACT FROM AUTHOR]

Silicone rubber is widely used as the housing material of composite insulator for outdoor high-voltage insulation. However, corona-induced aging in silicone rubber contributes to the ingress of water or moisture, which finally leads to fracture of a composite insulator. To investigate water transportation in silicone rubber helps to shed light upon the insulator fracture mechanism. In this work, water transportation behaviors in silicone rubber treated by different applied voltages of corona discharge were systematically studied using electrochemical impedance spectroscopy (EIS). Low voltages (3, 4 kV) of corona treatments result in growing value of fθmin in Bode phase angle, indicative of severer degree of surface delamination. After 5 kV of corona treatment, water can finally diffuse through the silicone rubber, and then induces double-layer capacitance (Cdl) and the charge transfer resistance (Rct) in EIS spectra. For high-voltage (≥6kV) corona-exposed samples, great number of surface cracks are observed, and then Warburg impedance appears in EIS spectra on initial period of immersion, suggesting a direct water transportation process. The polar groups generated in silicone rubber during corona exposure also facilitate this water diffusion process. Calculated water diffusion coefficients show a sharp increase between 5 and 6 kV, which serves as an indicator for percolation of cracks or defects in bulk of silicone rubber. [ABSTRACT FROM AUTHOR]

A series of silicone rubber samples with different cyclosiloxanes contents have been successfully prepared, and their hydrophobic recovery behaviors and mechanism were investigated in detail. The gas chromatography-mass spectroscopy technique after Soxhlet extraction was utilized to examine the low molecular weight siloxanes in the sample, SEM was used to observe the surface morphology of the silicone rubber influenced by plasma treatment, and contact angle measurement was applied to probe the hydrophobic recovery of the sample surface after plasma treatment at different storage temperatures. The storage time-dependent contact angle of water can be well fitted by the diffusion model calculated from Fick's second law. The results imply that the hydrophobic recovery of silicone rubber is related to the diffusion of low molecular weight siloxanes, while larger content or higher temperature can induce faster hydrophobic recovery. [ABSTRACT FROM AUTHOR]

In this paper, gas chromatography–mass spectrometry (GC–MS) and positron annihilation lifetime spectroscopy (PALS) were used to probe the changes of oligomers and the polydimethylsiloxane (PDMS) network in silicone rubber, after different durations of UVA/UVB irradiation. At the early stage (<300 h) of UVA/UVB irradiation, the concentration of D 4 -D 9 decreases. The o-Ps intensity of the extracted silicone rubber increases in the stage after UVB irradiation. These results indicate the crosslinking of oligomers into the PDMS network. After a long duration (>300 h) of UVB irradiation, D 4 was generated and the lifetime of τ 3 also increased, indicating the rupture of the Si-O bond in the PDMS network. These two aging processes were termed the post curing process and the chain session process. The new finding was that UVA could only induce the post curing process; UVB causes the rupture of the chemical bond in silicone rubber. Photons of UVB could break the C-H bond, and then trigger the backbiting decomposition of PDMS, breaking the Si-O bond, while the photons of UVA cannot. The fact that D 4 was generated after UVB irradiation can be used to evaluate the UVB stability of silicone rubber in the future. [ABSTRACT FROM AUTHOR]

Silicone rubber is the outer dielectric material of the composite insulator used in high‐voltage transmission lines. A study of the microstructure evolution during corona discharge could provide a new understanding of catastrophic fracture in composite insulators. In this paper, the corona degradation process of silicone rubber is divided into three stages based on the data obtained by slow positron beam and electrochemical impedance spectroscopy. In stage I, the S parameter decreases slightly in the low‐energy range, suggesting a crosslinking process in the polymeric "skin," and the silicone rubber holds its high value of resistance, Rpore. In stage II, the organic sample surface is gradually transformed into an inorganic one. Further cracking of the surface layer contributes to a dramatic increase in the sample porosity. At the beginning of water immersion, the Bode phase angle remains approximately −50∘ at low frequency, and a diffusion‐dependent Warburg impedance is observed. In stage III, a massive number of polar groups and inorganic particles are generated and the sample fails to repel water. The increase in the sample porosity is the main factor that leads to an ingress of water and failure of the material, while the generated polar groups are the secondary factor. [ABSTRACT FROM AUTHOR]

Abstract: A novel cross-linker octa[(trimethoxysilyl)ethyl]-POSS (OPS) based on a Vinyl-POSS derivative was firstly prepared via hydrosilylation chemistry. The novel POSS cross-linker was applied to hydroxyl terminated polydimethylsiloxane (HPDMS) matrix to prepare a series of novel RTV silicone rubbers. The chemical incorporation of novel POSS cross-linker into PDMS by hydrolytic condensation reaction was verified by attenuated total reflection (ATR) infrared spectroscopy. Morphology, thermal properties and mechanical properties of these novel RTV silicone rubbers have been studied by means of scanning electron microscope (SEM), thermal gravimetric analysis, universal tensile testing machine and dynamic mechanical analysis, respectively. The results displayed a pronounced enhancement effect of the novel cross-linker OPS on thermal stabilities and mechanical properties of RTV silicone rubbers as compared with the PDMS polymers prepared with tetramethoxysilane (TMOS), tetraethoxysilane (TEOS) and methyltrimethoxysilane (MTMOS) cross-linkers. Subsequently, the thermal decomposition of representative novel RTV silicone rubber in nitrogen was monitored by TGA coupled with real-time IR, and the degradation residues were also characterized by FTIR. It was found that the POSS cages were beneficial to increasing the degradation residues. The striking enhancements in thermal properties and improvements on mechanical properties could be attributable to the synergistic effect of the increasing dimensionality of cross-linking networks resulting from three-dimensional structure of OPS cross-linker, the plasticization of self-cross-linked OPS cross-linker and uniform distribution of POSS-rich domains. [Copyright &y& Elsevier]

[Display omitted] • The functional g-C 3 N 4 nanosheets are synthesized by sol-gel method. • The NO 2 resistibility of nanocomposite is evaluated by water diffusion via EIS. • Successful grafting silanes reduces diffusion pathways for NO 2. • The effect of the g-C 3 N 4 on chemisorption to NO 2 is calculated by DFT. • The synergy of physical barrier and chemisorption is achieved. In this study, the functional graphitic carbon nitride (g-C 3 N 4) nanosheets are prepared to enhance the NO 2 resistibility of room temperature vulcanized (RTV) silicone rubber. After corrosive gas NO 2 oxidation for 24 h, the water barrier ability of RTV nanocomposite coatings was evaluated by electrochemical impedance spectroscopy (EIS). At the optimal filler content of 0.3 wt.%, the impedance module | Z | of functional g-C 3 N 4 /RTV is 3☓108 Ωcm2 with the best coating integrality, compared with 2☓107 Ωcm2 for the functional graphene oxide (f-GO)/RTV and 6.45☓106 Ωcm2 for the pure RTV silicone rubber. The microstructure evolution in nanocomposite was investigated by positron annihilation lifetime spectra (PALS). The results indicate that the functional g-C 3 N 4 nanosheets improve the interfacial compatibility and reinforce the RTV matrix, which decrease the free volume for NO 2 diffusion. The density functional theory (DFT) calculations reveal that the triazine structures in the g-C 3 N 4 surface can effectively interact with NO 2 molecules. Simultaneously, silanes grafting promotes exfoliation and good dispersity of g-C 3 N 4. Due to highly decentralized two-dimensional functionalized g-C 3 N 4 with high aspect ratios, the significant synergy effect between physical barrier and chemisorption is achieved. [ABSTRACT FROM AUTHOR]