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Diffusion of low molecular weight siloxane from bulk to surface
Abstract
Silicone-based materials for outdoor insulators have the advantage
that low molecular weight (LMW) components migrate through the material
and coat the surface, thereby restoring hydrophobicity over a period of
hours. By measuring the IR absorption of siloxane migrating to the
silicone surface through a thin carbon coating, the time constant for
migration was calculated. According to the time dependence of
IR-absorbance, the time constant for migration ranged from 1.6 to 5.3 h
depending on alumina trihydrate (ATH) filler concentration
... In outdoor insulation, a hydrophobic surface can help to prevent water film formation on the insulation surface, which is crucial for minimizing leakage currents and partial discharge under severe weather conditions and in heavy polluted environments [1,2]. Instead individual water droplets are formed. ...
... Silicone rubber has an outstanding hydrophobicity among general insulation materials because its flexible Si-O backbone, allowing the substituted methyl groups to readily reorient to the surface thereby minimizing the surface energy [1]. ...
... Silazane is widely used in surface treating of the inorganic fillers, such as fumed silica. Surface hydroxyl groups can react with silazane molecules according to (1). ...
... Since XPS is a surface sensitive method, the silicone must have reached to the surfaces of the samples in order to be measurable. It has been reported that PDMS contains low-molecular-weight molecules that can easily diffuse through a 50 to 70 nm thick carbon coating within hours and restore hydrophobicity of a former hydrophilic surface [78]. Thus, this diffusion can be assumed to have taken place in our samples, since it can be inhibited but not completely prevented by the PPF with a thickness of 200 nm, and the partially cross-linked PEG layer, which had a thickness in the same range, as we reported earlier [50]. ...
... Thus, the C/O ratio and with it the polarity and degree of hydrophilicity can be assumed to stay close to the one of the starting material PEG. A more plausible explanation for the gradual loss of functionality is a combination of oxidative degradation, which is known to take place in grafted PEG surfaces and the aforementioned diffusion of siloxane molecules from the PDMS backing to the sample's surface, which alters the chemical composition and thus surface properties over time [72,75,78]. As the XPS results revealed the presence of silicon on the samples' surfaces, it can be assumed that siloxane diffusion plays a mayor role in the loss of hydrophilicity. ...
The concept of depositing solid films on low-vapor pressure liquids is introduced and developed into a top-down approach to functionalize surfaces by attaching liquid polyethylene glycol (PEG). Solid-liquid gradients were formed by low-pressure plasma treatment yielding cross-linking and/or deposition of a plasma polymer film subsequently bound to a flexible polydimethylsiloxane (PDMS) backing. The analysis via optical transmission spectroscopy (OTS), optical, confocal laser scanning (CLSM) and scanning electron microscopy (SEM), Fourier transform infrared (FTIR) and X-ray photoelectron spectroscopy (XPS) as well as by water contact angle (WCA) measurements revealed correlations between optical appearance, chemical composition and surface properties of the resulting water absorbing, covalently bound PEG-functionalized surfaces. Requirements for plasma polymer film deposition on low-vapor pressure liquids and effective surface functionalization are defined. Namely, the thickness of the liquid PEG substrate was a crucial parameter for successful film growth and covalent attachment of PEG. The presented method is a practicable approach for the production of functional surfaces featuring long-lasting strong hydrophilic properties, making them predestined for non-fouling or low-friction applications.
... In order for a surface to be superhydrophobic, it should have a water contact angle of 150 • or greater [6,7]. Silicone rubber (SR) as a polymer has a high hydrophobicity due to an inorganic backbone of Si-O that attaches to the organic groups (frequently methyl) [8][9][10][11]. SR is commonly used in many fields such as fabrication of electrical insulators [12], biomedicine, machinery, electronics and food processing [13] due to its excellent resistance, low toxicity [14], super transparency, high thermal stability [15], and hydrophobicity. ...
We report a simple low-cost novel method for coating an ultrathin hydrophobic film usable for different applications. A hydrophobic layer is fabricated on a TiO2 substrate by hydrolysis of silicone rubber evaporation. The hydrophobic –Si (CH3)3 groups bonded to the substrate by the reaction of hydrophilic ≡Si–OH groups with the hydroxyl groups on the TiO2 surface. In addition, the silicone film was introduced in a carbon-based perovskite solar cell as a tunneling passivation layer between the electron transport layer and the active layer to reduce interfacial and extraction losses. An improved perovskite layer with better crystallinity, larger grain size, and higher stability was achieved. The optimal energy conversion efficiency was 17.42% with FF, VOC, and JSC values of 73.76%, 1.1V, and 21.39mA cm-2, respectively.
... Siloxane and silanol vibration bands are always found in calcined silica xerogels and represent the final stage of sol-gel synthesis, denoting the hydrolysis reactions that produce silanol groups and condensation reactions that produce siloxane bridges. The strong absorption peak at 1270 cm −1 is the symmetrical vibration (υ s ) of the C−H group [36], while the peak at 760 cm −1 corresponds to the symmetrical vibrations (υ s ) of the Si−C group [37]. For both CH and Si−C groups, the adsorption increased with increasing MTES content in the silica matrix. ...
Silica membrane synthesis from tetraethoxysilane (TEOS) and methyltriethoxysilane (MTES) was carried out using ethanol-acetic acid solvent through a sol–gel process. The effects of TEOS-MTES mol composition of 100-0, 90-10, 75-25, 50-50, 25-75, 10-90, and 0-100 and calcination temperatures of 350 and 500 °C were studied. The water contact angle was used to examine the hydrophobicity of silica thin films. A close correlation between hydrophobicity and the presence of the Si−OH and C–H groups was obtained. Hydrophobicity was enhanced by increasing the C−H/Si−O−Si ratio and decreasing Si−OH/Si−O−Si ratio. Silica membrane material maintained hydrophobic properties up to 400 °C. Thermal stability went up with the rise in MTES content. All silica xerogels exhibited microporous character with decreased pore volume and surface area with lower TEOS content. The pervaporation technique systematically studied the TEOS-MTES silica membrane’s desalination performance using various saltwater concentrations and feed temperatures. The TEOS-MTES composition played an essential role in determining the membrane’s character and performance, salt rejection, and water fluxes. The high TEOS content led to high water flux. Alternatively, a high MTES content led to a high salt rejection. All membranes provided good salt rejection with values up to 99%, especially those with high MTES contents.
Graphical abstract
... The diffusion time constant for LMW siloxanes migration directly reflects the rate of hydrophobicity recovery in silicone rubber. It has been found in various studies that the diffusion time constant increases with increasing micro-ATH filler concentrations in silicone rubber, and higher ATH concentrations are responsible for slowing down siloxanes diffusion [44,45]. This could be due to two reasons; first, the PDMS cross-section available for siloxanes migration decreases with increasing filler levels. ...
This paper discusses the state of the art in the application of self-healing silicone-based
materials for outdoor high-voltage insulation. Both the dynamic behavior of the dimethyl side groups of silicone rubber and the diffusion of a bulk siloxane to maintain low surface energy are respectively reported as intrinsic mechanisms responsible for the self-healing of silicone rubber. Localization, temporality, mobility, and the type of synthesis are the aspects defining the efficiency of the self-healing ability of silicone rubber. In addition, the deterioration of the self-healing ability with filler loaded into silicone rubber insulation housing composites is discussed. Taking the self-healing property into consideration among the other properties of silicone rubber insulators, such as tracking and erosion resistance, can be a useful design practice at the material development stage. Hydrophobicity retention, recovery, and transfer measurements are discussed as useful indicators of the self-healing ability of silicone rubber. Nevertheless, there remains a need to standardize them as design tests at the material development stage. The paper is intended to shed the light on the hydrophobicity recovery, a key material design parameter in the development of silicone rubber outdoor insulating composites, similar to the tracking and erosion resistance.
... The band can be separated into two bands at ~ 1030 cm −1 and 1080 cm −1 , indicating linear and cyclic Si-O-Si, respectively [6]. The presence of the Si-O-Si peak is supported by the presence of a peak at 800 cm −1 , while the small peak at 1260 cm −1 is associated with symmetrical vibrations (υs) of the Si-CH 3 bond [37]. The existence of a peak at 1260 cm −1 for uncalcined and 350 °C-calcined xerogel samples corresponds to the Si bonded to a CH 3 group. ...
In this study, hydrophobic silica layers were synthesized through the sol–gel method derived from the silica precursor, methyltriethoxysilane (MTES), at various pH and calcination temperatures. Coating silica on ceramic plates significantly increases the surface hydrophobicity of the ceramic plates. The increment in hydrophobicity comes from the surface roughness and the hydrophobic group –CH3 derived from MTES. Increasing pH causes the hydrophobicity of the silica film to escalate. In the MTES silica film calcined at 350 °C, a change in pH from 1.97 to 9.66 enhanced the water contact angle from 92° to 120°, while increasing the calcination temperature decreased surface hydrophobicity. MTES silica film can maintain hydrophobic properties up to 420 °C. Mid pH (approx. 4.66) yields the silica material with the highest thermal stability. Gas sorption analysis shows that the resulting material has low porosity, surface area, and pore volume.
... Silicone rubber has ability to retrieve its hydrophobicity when a pollution layer is formed on its surface JongsooKim et al. (2000), Farzaneh (2011). It can attribute the property of hydrophobic recovery of the silicone rubber serves to transmit a low molecular weight siloxane from bulk to surface, and this layer is adsorbed (physically or chemically) on the contaminant Homma et al. (1999). Contaminant particles are encapsulated by low molecular weight siloxane layer which improves moisture absorption resistance of contaminant particles (Figure(3)). ...
Silicone rubber (SIR) is superior to natural rubbers due to having special features like high chemical stability, heat, abrasion and ozone resistance. Global markets show huge attention to produce and use silicone rubber in many applications such as aerospace, automobile, construction, electric insulators and medical industries. Although silicone rubber is hugely utilized in high voltage insulators, it shows low properties such as mechanical properties. Therefore, fillers are added to silicone rubber in order to reducing cost and improving the surface hydrophobicity, thermal and/or electrical conductivity, thermal and/or electric insulation and mechanical properties. This paper reviews on structure, types, general properties, manufacturing methods, applications of SIR, fillers and silicone rubber nanocomposites. It is focused on the popular micro/nanoparticles for dielectric applications. Organic montmorillonite (OMMT) represents one of the most important fillers that are used for dielectric applications. OMMT is added to SIR to improve the cost and electric and thermal insulation and mechanical properties. Tensile properties of SIR/OMMT nanocomposites increased with increasing the content of OMMT. SIR/OMMT nanocomposites with smaller OMMT size showed the highest compression strength and the lowest tear strength. SIR/OMMT Nanocomposites with high content of OMMT displayed greater thermal stabilities, flame retardant properties, dielectric permittivity and dielectric loss.
... Thus, the closer to the surface the LMW components are, the faster the material recovers. It must be noted that in aged material the amount of LMW components close to the surface is exhausted, thus they emerge from large depth, which requires more time [19,26]. ...
Silicone rubber (SIR) insulators are known to maintain their surface hydrophobicity even under severe pollution conditions in contrast to the other composite insulator materials used at the last decades. This critical advantage of silicone rubber insulators has made them dominant in high voltage power systems despite the fact that there are other composite materials with better static hydrophobicity. In service conditions, priority is given to the dynamic performance of hydrophobicity due to the unpredictable environmental pollution conditions. This dynamic performance of silicone rubber insulators is also known as hydrophobicity transfer mechanism. In literature, the hydrophobicity transfer mechanism of silicone rubber is related to the reorientation of methyl-groups and the existence of low molecular weight components. However there are many parameters which can change the effectiveness of this mechanism. Some of them referred to the ageing effects on the material structure. Thus it is of great importance to investigate the hydrophobicity transfer mechanism of field aged composite insulators. For this reason a new experimental procedure is introduced based on Cigre TB 442. The results of field aged insulators are compared to that of a new SIR insulator revealing the superiority of silicone rubber even after 17 years of field ageing.
... Silicone rubber has ability to retrieve its hydrophobicity when a pollution layer is formed on its surface JongsooKim et al. (2000), Farzaneh (2011). It can attribute the property of hydrophobic recovery of the silicone rubber serves to transmit a low molecular weight siloxane from bulk to surface, and this layer is adsorbed (physically or chemically) on the contaminant Homma et al. (1999). Contaminant particles are encapsulated by low molecular weight siloxane layer which improves moisture absorption resistance of contaminant particles (Figure(3)). ...
Silicone rubber (SIR) is superior to natural rubbers due to having special features like high chemical stability, heat, abrasion and ozone resistance. Global markets show huge attention to produce and use silicone rubber in many applications such as aerospace, automobile, construction, electric insulators and medical industries. Although silicone rubber is hugely utilized in high voltage insulators, it shows low properties such as mechanical properties. Therefore, fillers are added to silicone rubber in order to reducing cost and improving the surface hydrophobicity, thermal and/or electrical conductivity, thermal and/or electric insulation and mechanical properties. This paper reviews on structure, types, general properties, manufacturing methods, applications of SIR, fillers and silicone rubber nanocomposites. It is focused on the popular micro/nanoparticles for dielectric applications. Organic montmorillonite (OMMT) represents one of the most important fillers that are used for dielectric applications. OMMT is added to SIR to improve the cost and electric and thermal insulation and mechanical properties. Tensile properties of SIR/OMMT nanocomposites increased with increasing the content of OMMT. SIR/OMMT nanocomposites with smaller OMMT size showed the highest compression strength and the lowest tear strength. SIR/OMMT Nanocomposites with high content of OMMT displayed greater thermal stabilities, flame retardant properties, dielectric permittivity and dielectric loss.
... 4 SiR is also called as poly dimethyl siloxane since its chemical structure contains organic hydrocarbon methyl group (-CH 3 ) as a functional substituent and inorganic siloxane (Si-O) as the backbone. 5 Ethylene propylene diene monomer (EPDM) has good mechanical properties since it possesses a saturated structure with polymer backbone and also it is inexpensive compared to SiR. The chemical structure of EPDM is a terpolymer which contains ethylene, propylene and diene monomer. ...
The aim of the research is to prepare a polymer composite material for high-voltage cable insulation purposes. Many researchers have blended a silicone rubber (SiR) and ethylene propylene diene monomer (EPDM) at different ratios to identify the composition which possesses excellent electromechanical properties. SiR-EPDM blends in 90:10 and 10:90 ratios were taken up for this study. To these polymer blends, metallic filler titanium dioxide (TiO 2 ) with different densities have been used as the filler materials. The electromechanical performance and the physico-chemical behaviour of SiR-EPDM blends with filler addition were compared with SiR-EPDM blends of the same blend ratio without filler. The electromechanical parameters were measured according to the International Electrotechnical Commission (IEC) and ASTM standards. The surface morphology and filler dispersion were examined by the scanning electron microscopy images. The elemental compositions of the samples were obtained from the energy dispersive X-ray analysis. The changes in electromechanical behaviour of the samples were analysed from Fourier-transform infrared spectroscopy results. The investigations reveal that 5 wt% of high-density TiO 2 -filled SiR-rich blend possesses the best electromechanical performance of all prepared samples.
... In the region between 20 and 35 min, the baseline shows a broad hump which represents a large number of siloxane derivatives (Fig. 3) eluted at the same time, giving rise to unresolved peaks [8]. The presence of a high number of siloxane species have been previously reported as a fraction of silicone oligomers [24][25][26][27]. Siloxanes are not particularly reactive materials, therefore their reactivity and potential damage in museum artifacts has not been evaluated. ...
Some volatile organic compounds (VOCs), generated and accumulated in exhibition cases, are capableof inducing degradation in historical artifacts. In this context, it is of special importance to distinguishbetween VOCs emitted by the construction materials and/or the historical objects, with the ultimate goalof proposing an appropriate mitigation strategy. To pursue this goal, a comprehensive analysis basedon solid phase microextraction (SPME) coupled to gas chromatography-mass spectrometry (GC-MS) hasbeen optimized. The sampling setup used in this study does not require any modification of the exhibitiondesign, and the testing remained nearly invisible and unobtrusive to museum visitors. This methodologyhas been applied to an issue encountered at the National Museum of the American Indian (Smithso-nian Institution), where an unpleasant smell was detected in airtight exhibition cases already on displaycontaining sensitive historical objects. The analysis of the volatiles emitted by construction materialsallowed for identification of characteristic markers emitted by each material, and the ability to iden-tify their overall contribution in the exhibition case. Via this method, SPME-GC-MS confirmed via thedetection of some specific markers that the unpleasant smell was released by one of the constructionmaterials and had accumulated in the exhibition case. In addition, due to the measurements taken indifferent positions around the exhibition cases, results were useful to point out the range of efficiency ofthe sorbent material placed in some of the exhibition cases as a first attempt of mitigation technique
... In our study, the peak current density for neat SR was higher than the SR composites indicating a decrease in the diffusion rate of LMW due to the presence of the filler. This is also in good agreement with Homma et al. findings, who showed similar response with increase in the amount of SiO 2 in SiO 2 /PDMS composites [37]. ...
The present research work was aimed at improving the high field dielectric properties of silicone rubber (SR) composites used for power cable insulation. Liquid silicone rubber composites were developed using two types of clays as filler materials; organically modified montmorillonite (ommt) and calcined, surface treated kaolin with the commercial name Translink 37 (T37). The results revealed that the surface chemistry of the clay had a distinct effect on both dispersion as well as electrical performance of silicone rubber-clay composites. We observed that surface treated kaolin (T37) were well dispersed in the SR matrix, but had adverse impact on the crosslinking kinetics of liquid silicone rubber resulting in higher dielectric loss at low frequencies. On the other hand, organically modified clay showed poor dispersion in the silicone rubber matrix while exhibiting much lower dc conductivity characteristics at high temperature. The agglomerated ommt macroparticles introduced the interfacial traps in SR matrix and charge carriers migration was limited within the matrix.
... Low surface energy is one of the most demanding properties in insulators where the SIR insulator is dominating over all others insulators in the power system. The hydrophobicity of SIR has widely accepted in today's world of insulators [1]- [3]. SIR is the superior material which is being used mostly over the glass and porcelain [4]- [6].However, the material used to degrade after long time exposure to the environmental conditions such as acid rain, UV rays, dust and salt fog. ...
... The SIR has become the first choice by the high voltage power apparatus utilities. The hydrophobicity of SIR has widely accepted in today's world of insulator [1]- [3]. However, the material got degraded after long exposure to the environmental conditions such as acid rain, UV rays and salt fog. ...
... The inherent hydrophobicity of PDMS is not desirable in applications that require hydrophilicity especially in contact with blood and body fluids [17,65]. Therefore, in the present work, some efforts were performed to make the PDMS surface more hydrophilic and to suppress the hydrophobic recovery due to its superhydrophobicity and low glass transition temperature. ...
... The inherent hydrophobicity of PDMS is not desirable in applications that require hydrophilicity especially in contact with blood and body fluids [17,65]. Therefore, in the present work, some efforts were performed to make the PDMS surface more hydrophilic and to suppress the hydrophobic recovery due to its superhydrophobicity and low glass transition temperature. ...
Despite the distinct advantages of poly(dimethylsiloxane) (PDMS) for biomedical applications, because of its hydrophobic nature, suffers from non-specific protein adsorption and platelet adhesion and activation when used as a blood-contacting material. To confer hydrophilicity and biomolecules repelling characteristics, well-defined and high-density poly(2-hydroxyethyl methacrylate) (PHEMA) brushes are synthesized via surface-initiated atom transfer radical polymerization (SI-ATRP) on the PDMS substrate. First, PDMS surface is activated using an ultraviolet/ozone (UVO) wet treatment in water media to introduce hydroxy moieties without scarifying the surface property resulting a crack-free SiO2 surface. Then, 3-(2-bromoisobutyramido)propyl(trimethoxy)silane (BrTMOS), the active ATRP initiator, is immobilized on the UVO-treated PDMS surface to prepare a thin layer of hydrophilic PHEMA brush on PDMS substrate exhibiting excellent protein and platelet resistance. PHEMA brushes supply a biomimetic feature by combining antifouling properties due to hydrophilic characteristic with bioactive properties resulted from the presence of a high density hydroxy groups, which are subsequently used for biomolecules conjugation. The results indicate that grafting of PHEMA chains on the PDMS surface enhances the surface wettability which leads to a decrease in non-specific protein adsorption and platelet adhesion compared to the bare PDMS substrate. The adhered platelets on the PHEMA-tethered PDMS substrate maintain their normal round morphology. Furthermore, the conjugated gelatin macromolecules onto the tethered PHEMA chains promote the adherence and growth of human umbilical vein endothelial cells via ligand-receptor interactions.
... The FTIR spectra of the synthesized MTMS silica xerogel are shown in Fig. 7. Fig. 7 confirms that the pH variation and the calcination temperature affected the position, shape and intensity of the MTMS silica xerogel absorbance. The emerging peaks are characteristic for silica polymers located at wave numbers between 1200-750 cm -1 , i.e. (i) wave numbers of 1150-1000 cm -1 indicated the presence of the symmetric and asymmetric vibrations of Si-O-Si [32], (ii) the shoulder region at wave number of 955-835 cm -1 in the form of weak absorption overlapping with strong adsorption was attributed to the Si-OH vibration, (iii) the wave number of 865-750 cm -1 was assigned to Si-C group which was easily recognized because of the strong and sharp absorption peaks [33], and (iv) a small peak of 1260 cm -1 corresponded to the symmetrical vibration of the C-H group [34]. [31], the difference in calcination temperature resulted in higher Si-OH contents at higher temperatures. ...
This study investigated the synthesis and characterization of MTMS hydrophobic silica prepared by sol-gel method. In principle, silica xerogels and silica thin layer were obtained by reacting MTMS in ethanol solvent in some pH variations. The MTMS solution was used to modify the surface of the ceramic plate by dipcoating method to further be calcined at two different temperatures of 350°C and 500°C. The silica xerogels were analysed by FTIR, TGA-DSC and GSA to determine functional group characteristics, thermal properties and pore morphology respectively. Meanwhile, the silica thin layers were analysed their hydrophobic properties using water contact angle measurement and surface roughness determination using SEM. The results showed that the higher the pH used in the MTMS solution, the higher the resulting contact angle. The highest contact angle was obtained at pH 8.12 which reached 94.7° and 79.5° for silica thin layer calcined at 350°C and 500°C, respectively. The TGA results indicated that the methyl group survived up to 400°C and disappeared at 500°C which had implications on silica thin layer hydrophobic nature. GSA result exhibited that the silica xerogel had a close structure with a very low pore volume. While the SEM-EDX results displayed that the silica thin layer prepared at acidic pH had smoother surface morphology and became rough when prepared at an alkaline pH.
... Deposit of non-soluble substances derived from hydrophilic inorganic compounds such as SiO2 and Al2O3 and called dust affects to decrease the hydrophobicity of SIRs. Even though SIRs are covered with dust, they can recover the reduced hydrophobicity because of migration of low-molecular-weight (LMW) silicone chains onto the surface of deposited dust[15,16]. SF test involving with dust deposit can evaluate the ability of hydrophobicity recovery as well as that of hydrophobicity stability.Table 1. ...
The guideline and procedure to select the outdoor silicone rubbers having the best electrical performances from candidates by using slab samples and inclined-plane and salt-fog tests are introduced. Inclined-plane test, which eliminates the material hydrophobic effect, can make evaluations which do not correlate with actual outdoor use to silicone-based materials. However, good understandings on it and the combination with salt-fog test enable an appropriate and valid material selection. It is shown that proposed modified salt-fog method which involves dust deposit has a possibility to produce significant differences among excellent silicone rubbers and to evaluate their performances in considerably severe conditions. The recommended procedure will be helpful for users and manufactures in evaluating and screening silicone rubbers for outdoor insulation.
... The increase in the concentration of carbon during the first two weeks after deposition could be explained by the surface of the coating becoming more organic. This could be the result of the unreacted precursor, short chain polymer fragments and oligomers diffusing onto the surface as it has been observed for siloxanes synthesised in wet chemistry processes [18] . Also, the C 1s and Si 2p fine scan spectra showed no measurable change in the shape of the peaks during the test suggesting that the change in carbon content is unlikely due to atmospheric contamination. ...
In this work, the post-synthesis changes of chemistry and surface properties of the siloxane coatings synthesized via Plasma Enhanced Chemical Vapor Deposition (PECVD) are investigated. The study shows that in contrast to the volume of the siloxane coating, which undergoes oxidation and increase in crosslinking density during several days after deposition, the surface of the coating displays different effects. Further analysis shows that this is the result of short polymer fragments diffusing onto the surface, affecting wettability of the surface and adhesion. It has also been observed that these short polymer fragment may be removed and will in time replenish, giving the coatings a regenerative nature.
... Thus, the closer to the surface the LMW components are, the faster the material recovers. It must be noted that in aged material the amount of LMW components close to the surface is exhausted, thus they emerge from large depth, which requires more time [19,26]. ...
Silicone rubber (SIR) insulators are known to maintain their surface hydrophobicity even under severe pollution conditions in contrast to the other composite insulator materials used at the last decades. This critical advantage of silicone rubber insulators has made them dominant in high voltage power systems despite the fact that there are other composite materials with better static hydrophobicity. In service conditions, priority is given to the dynamic performance of hydrophobicity due to the unpredictable environmental pollution conditions. This dynamic performance of silicone rubber insulators is also known as hydrophobicity transfer mechanism. In literature, the hydrophobicity transfer mechanism of silicone rubber is related to the reorientation of methyl-groups and the existence of low molecular weight components. However there are many parameters which can change the effectiveness of this mechanism. Some of them referred to the ageing effects on the material structure. Thus it is of great importance to investigate the hydrophobicity transfer mechanism of field aged composite insulators. For this reason a new experimental procedure is introduced based on Cigre TB 442. The results of field aged insulators are compared to that of a new SIR insulator revealing the superiority of silicone rubber even after 17 years of field ageing.
Hydrophobic silica thin films are produced from sodium silicate and methyltrimethoxysilane (MTMS). CTAB has a hydrophobic group that can increase its hydrophobicity. The purpose of this study was to examine the effect of adding CTAB to silica thin film and xerogel with sodium silicate and MTMS as a precursor. Thin film and xerogel were obtained using the sol-gel and dip-coating methods. The results showed that the addition of CTAB affected the hydrophobic properties of the thin film and xerogel samples. The more CTAB added, the greater the water contact angle, with a limit for adding CTAB of 0.064 g. The water contact angle was obtained for the highest addition of CTAB, with values of 91°, 105.4°, and 107.1° respectively for thin films, non-calcined xerogel, and calcined xerogel. The FTIR results showed that in the calcined and non-calcined xerogel samples, the Si-OH/Si-O-Si ratio tended to decrease.
The anti-pollution flashover characteristic of composite insulators is the main indicators to evaluate their electrical performance, and the pollution flashover voltage can directly characterize the anti-pollution flashover characteristics of the insulators. This paper proposed an on-line detection method for the pollution flashover voltage of composite insulators. Samples of composite insulator shed with different hydrophobicity were prepared by using NaOH solution with pH value of 13 and temperature of 80 °C. According to the distribution of water droplets on the surface of the sample, the characteristic parameter of the number of water droplets was proposed. The samples with different characteristic parameters were smear-stained and used for the pollution flashover voltage test. The relationships between the number of water droplets on the sample surface and the pollution flashover voltage and leakage current were obtained, which can provide guidance for the evaluation of the operation state of composite insulators.
Silicone rubber micro nanocomposite specimens were aged in acidic (acid rain and HNO3) and saline (NaCl) solutions at 90˚C for 24 h and subsequently placed in the open air at room temperature to monitor its recovery characteristics. The specimens aged in acidic and saline solutions with high conductivity have shown better performance than the samples aged in acidic and saline solutions with normal conductivity. Lesser reduction in hydrophobicity and surface roughness after ageing and marginally faster recovery of hydrophobicity and surface roughness were observed for the silicone rubber specimens aged in NaCl solution followed by HNO3 and acid rain solutions. The micro nanocomposites have reflected the least water intake and lesser surface degradation along with better hydrophobicity compared to pure silicone rubber specimens. Surface morphological changes in aged silicone rubber specimens were analysed in detail at various decomposition levels, by employing wavelet transform to surface profile signal obtained through Atomic Force Microscopy (AFM). The fractal dimension and lacunarity parameters evaluated for the surface profile data of aged silicone rubber composites were found to increase marginally, indicating marginal increment in the complexity and the fractal surface density.
Digital light processing (DLP) 3D printing is advantageous in high printing efficiency and printing resolution for fabricating complex structures across various applications. However, the layer‐by‐layer curing manner of DLP leads to weak interlayer adhesion and the anisotropic mechanical properties of printed objects. Here, linear polymers are introduced into commercial resins to weld the interlayer by the diffusion and entanglement of linear polymers after DLP printing via heat treatment. This introduction of linear polymers not only shows a strengthening and toughening effect on the printed objects, but also has no negative impact on the DLP printability. The tensile strengths of objects containing 4.7wt% polycaprolactone can reach up to ∼500% of that of neat samples in any printing direction. This simple strategy by adding linear polymers into printing resins provides an effective access to prepare DLP printed objects with improved mechanical properties as well as ensure printing resolution and printing efficiency. This article is protected by copyright. All rights reserved
Silica thin film preparation was carried out from dimethyldimethoxysilane (DMDMS) and tetraethylorthosilicate (TEOS) precursors using the sol-gel method, and the effect of pH on the character of the resulting silica membrane was also studied. The solution pH was varied into 2.21; 3.92; 4.98; 6.44; and 7.21. The resulting xerogel was analyzed using Fourier-transform infrared spectroscopy, Thermogravimetric Analysis, and Gas Sorption Analysis. While the silica thin layer was measured for its hydrophobic character and surface morphology using Scanning Electron Microscope. The presence of DMDMS-TEOS silica thin film had a considerable influence on the hydrophobic nature of the ceramic plate surface. The membrane prepared under neutral conditions produced desirable properties, such as higher thermal stability, greater pore volume, thermal stability, and higher hydrophobicity. For the desalination process, the obtained silica membrane showed excellent separation performance (>99%), especially at low salt concentrations. When the pH increased, the water flux also increased. The thin films worked well for desalination through a pervaporation mechanism.
The synthesis of the hydrophobic silica layer was conducted using the sol–gel method by reacting silica precursor trimethylchlorosilane (TMCS) and tetraethylorthosilicate (TEOS) with variations in the precursor composition ratio (TMCS:TEOS) of 10:90, 25:75, 50:50, 75:25, and 90:10 and then calcined at various temperatures. The TMCS‐TEOS silica thin layer has a significant influence on the surface hydrophobicity of the glass. The higher the number of TMCS precursors, the greater the contact angle produced. The highest contact angle produced was 98° at a TMCS‐TEOS ratio of 75:25 and without calcination. The TMCS‐TEOS silica thin layer contact angle decreased with increasing calcination temperature. The hydrophobicity of the TMCS‐TEOS silica thin layer could withstand temperatures up to 300°C. Quantitative analysis of FTIR spectra's peak area shows that with the increasing TMCS, the ratio of silanol/siloxane decreases. Gas sorption analysis shows that the TMCS‐TEOS thin layers' pore sizes are in the mesoporous region, and the pore shapes are cylindrical at all TMCS:TEOS ratios and calcination temperatures.
We report the use metal organic frameworks for the selective inclusion of nucleic acids with different size, shape and capability of conformational transition. Three MOFs, Co-IRMOF-74-II, -III, and -IV, composed of Co2+ and organic linkers (linker-II, -III and -IV), respectively, were used for nucleic acid inclusion into their pores from solution, which is a spontaneous process from disordered free state to restricted ordered state. The pores of these MOFs were precisely controlled to provide selectivity between different structural nucleic acids. These MOFs preferentially adsorb flexible single strand nucleic acids due to spatial size matching selection and preferentially adsorb unstable rigid structural nucleic acids with larger cross-section size owing to conformations changes. In addition, the transcribed RNA enters the MOF pores in a directional manner, giving priority to the 3' end entering the MOF pores. Furthermore, under complicated mixtures of various structures, flexible single strand nucleic acids and unstable rigid structure nucleic acids were selectively adsorbed, leaving stable secondary structure nucleic acids in the solution, which can be collected to characterize the structure. It indicates that MOF materials has a great potential for application in the field of enrichment and determine all type of structural nucleic acids, especially in the field of monitoring all structural RNA due to the dynamic change of RNA conformations.
Silicone rubber (SIR) samples are fluorinated using a F2/N2 mixture at different temperatures. Physicochemical characteristics are analyzed by attenuated total reflection infrared spectroscopy, X‐ray photoelectron spectroscopy, and scanning electron microscopy. Basic properties of the fluorinated surface layers are investigated, and the intrinsic alternating current (AC) flashover performance and tracking resistance of the surface fluorinated samples are evaluated. The flashover test results indicate that the fluorination leads to an improvement in AC flashover performance of the SIR material, as a result of the competition between the favorable effect of the increase in surface conduction and the adverse effect of a likely increase in surface layer permittivity. The increase in AC flashover voltage is associated with the voltage‐increasing mode of the flashover test. The flashover test results also show an improved tracking resistance of the SIR material by the fluorination, and the improvement is especially significant for the fluorinated surface layer that has less cleavage of SiC bonds or a good compactness as well as a large thickness. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019, 137, 48556.
This study shows, for the first time, the synthesis of a carbonised pectin templated silica thin-film material. The aim of this work is to investigate the vibration of carbon groups of pectin–silica xerogels using infrared spectroscopy and curve-fitting software. Thin films were prepared using a two-step acid–base catalysed sol-gel method, employing tetraethyl orthosilicate as the main precursor. Various concentrations of pectin (0.1 wt%, 0.5 wt% and 2.5 wt%) were used as templating agents. These were firstly dissolved in glycerol, and mixed with the silica thin-film material. The sols was then dried in an oven at 60°C for 12 h, and calcined in air using rapid thermal processing at 300°C and 400°C for 1 h. The results show that the concentration of carbon groups for samples calcined under 400°C is higher than that of samples calcined at 300°C – especially in the region of 800 cm⁻¹ for silica-carbon. This clearly shows that the calcination temperature influences the polymerisation of carbon group templates in the silica matrices.
Heating, ventilation, and air conditioning systems represent nearly half of building energy usage in the United States. Recent building regulations requiring increased air turnover within buildings will result in even greater air conditioning usage. To alleviate these concerns, membrane-based heat-and-moisture exchangers known as energy recovery ventilators have been developed to reduce the energy expenditure of air conditioning systems by pre-conditioning incoming air through sensible and latent heat transfer with building air exhaust. In this work, we propose and validate a manufacturing process design for channel lamination based on surface mount adhesives capable of meeting the process requirements of a membrane-based ventilator. In particular, the device requires rib supports with height-to-width aspect ratios greater than one. The proposed manufacturing process design is capable of meeting this process requirement by stretching the adhesive after initial adhesive tacking. A manufacturing process flow diagram, a machine tool specification and a cost model are proposed for meeting process requirements. A set of design constraints are developed detailing the adhesive requirements necessary for meeting requirements. The cost model is validated by building a sub-scale mesochannel array demonstrating the ability to meet process requirements. Results show that additional work is needed to validate the curing step but that the process is capable of producing a mesochannel array with an acceptable level of variation.
Silicone rubber is widely used in power system insulation for its hydrophobicity and hydrophobicity transfer to prevent pollution flashover. The influence of low molecular weight (LMW) siloxanes on hydrophobicity of contaminated silicone rubber after plasma jet treatment was investigated in this study. Experimental results show that plasma jet treatment for several seconds can significantly improve the hydrophobicity of contaminated silicone rubber. LMW siloxanes play an important role in the quick recovery of hydrophobicity induced by plasma jet treatment. Extraction of the LWM siloxanes from the silicone rubber decreases the contact angle after plasma treatment. Fourier transform infrared (FTIR) spectroscopy shows that hydrophobic groups appear on the surface of the contamination layer. The hydrophobicity improvement of contaminated silicone rubber results from LMW siloxanes transferred to the contamination layer during plasma treatment and the LMW siloxanes are mostly original uncrosslinked siloxanes in the silicone rubber. The molar mass distributions of the LMW siloxanes in hydrophobic contamination of treated specimens were similar with naturally transferred ones. X-ray photoelectron spectroscopy shows that an inorganic layer appears after plasma aging, which obscures the transfer of LMW siloxanes and reduce the hydrophobicity during plasma treatment.
3D printed parts prepared by fused deposition modeling (FDM) are well known to exhibit large anisotropic mechanical properties. More precisely an object printed with layers orthogonal to the print bed (transverse) are significantly weaker than those printed parallel (longitudinal). The reason for this behavior is due to poor diffusion and entanglement of chains across the interlayer interface thus resulting in a weak interlayer bond. To combat anisotropy in FDM printed parts, our group has utilized bimodal blends of a chemically identical low molecular weight surface segregating additive (LMW-SuSA) blended with a bulk, commercially available poly(lactide) (PLA). Drastic improvements in the interlayer adhesion and a more isotropic part is realized with the introduction of the LMW-SuSA. To expand our understanding of the mechanism responsible for this improvement, we report the introduction of LMW-SuSAs of miscible styrene-co-acrylonitrile (SAN), poly(methyl methacrylate) (PMMA) and immiscible PLA to ABS and their impact on the mechanical properties of printed FDM parts. Decreases in the anisotropy of mechanical properties of ABS blends containing SAN (8.5k, 33k and 75k), PMMA (33k, 67k and 100k) and PLA (33k-3 arm and 220k) are tested. With the addition of 33k PMMA and 33k-3 arm PLA to ABS, the transversely oriented parts maximum stress increases by 40% and 25% respectively. A significant improvement in isotropy in the modulus is also observed. Interestingly, LMW-SuSAs of SAN do not improve the isotropy of the part. More importantly, experiments utilizing energy dispersive x-ray spectroscopy (EDS) confirm the surface segregation of LMW PMMA and PLA to the interfilament interface, indicating that improvements in layer adhesion are a result of increased diffusion and entanglement of chains across the interlayer interface.
Although flashovers on snow-capped insulators have been discussed since at least the 1950s, one of the advanced topics recently is flashovers following harsh contaminated snowstorms.
Engineered nanocomposites with tailored dielectric and hydrophobic characteristics are highly desirable for miniaturized electronics. In this context, we have fabricated acrylonitrile butadiene styrene (ABS)/NiFe2O4 nanocomposites by facile solvent casted and low-volume high-pressure air-atomized (LVHPAA) techniques. The developed nanocomposites consisted of phases such as amorphous ABS, and crystalline nickel–ferrite phases were examined via x-ray diffraction technique. Fourier transform infrared spectroscopic measuration was used to describe ABS polymers, nickel–ferrite oxide constituents, and their interactions. The effect of the space-charge polarization mechanism between particulates and the polymer (10¹–10² Hz and 10–40 wt.%) was analyzed via impedance spectroscopy, which is further augmented by the Maxwell–Wagner Sillars polarization hypothesis. Also, the subsequent oriental relaxation phenomenon (10³–10⁷ Hz) was analyzed. Moreover, at ∼ 10⁷ Hz for all wt.% (10–40 wt.%), the incremental permittivity attributed oriental resonance phenomena was examined. In addition, the developed nanocomposites DC-conductivity attributed micro/nano-capacitors mechanism, and the AC-conductivity realized reorientational hoping mechanisms were scrutinized. The cole–cole representation of a nanocomposite that explained relaxation oriented insulating characteristics was also elucidated. The hydrophobicity of developed composites was characterized via atomic force microscopy (AFM) and contact angle goniometry. The AFM analysis showed a uniform textured surface morphology with the LVHPAA technique, which renders superior hydrophobic characteristics due to the process induced nano-needle generated roughness factor. The investigation results explain the improvement in the dielectric and hydrophobic characteristics of nanocomposites obtained by the LVHPAA technique. Therefore, these ABS/NiFe2O4 nanocomposites could be a possible functional material for miniaturized electronic applications.
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This paper investigates the effect of aluminum hydroxide (ATH) content on the free volume and surface recovery property of polydimethylsiloxane (PDMS)–based silicone rubber containing low-molecular-weight siloxanes. With increasing ratio of ATH up to 43.1 wt %, the concentration of cyclic siloxanes (Dn = [(CH3)2SiO]n, n = 4–12) in the PDMS matrix increases remarkably, indicative of a spacing effect of ATH particles on the crosslinking of PDMS chains. When more ATH is added, the concentration of D4–D12 began to decrease. PDMS network variation is verified by free volume size corresponding to τ3 in positron annihilation lifetime spectroscopy. The o-Ps intensity decreases linearly with ATH content. Data obtained from X-ray photoelectron spectroscopy suggest the surface recovery property is weakened by ATH. This process is dominated by the amount of free volume holes in the sample. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018, 135, 45803.
Parts prepared by the fused deposition modeling (FDM) 3D printing process suffer from poor interfacial adhesion between layers. This is due to poor diffusion of the very large and slow polymer chains across the inter-filament interface. To address this issue, we have developed the use of a bimodal blend of poly(lactide) (PLA) comprised of a series of synthesized low molecular weight PLA components (8.5 k, 50 k, and 100 k) added to a commercially available PLA (220 K). Tensile testing results indicate that when the LMW additive is of a sufficient length, the maximum stress and modulus in the part printed orthogonal to the print head (transverse) is significantly improved. More specifically, this behavior is observed where increased diffusion and increased entanglement of chains across adjacent layers occurs. The extent of crystallization at various stages of processing is also analyzed and indicates no correlation between the mechanical properties obtained and extent of crystallinity.
Periodic microswelling structures were photochemically induced on a silicone rubber surface using a 193 nm ArF excimer laser. Microspheres made of silica glass (SiO2) of 2.5 µm diameter were aligned on the silicone rubber surface during laser irradiation; the laser beam was focused on the silicone surface underneath each microsphere. The height and diameter of the formed microswelling structures were found to be controllable by changing the Ar gas flow rate, single-pulse laser fluence, and laser irradiation time. The chemical bonding of the laser-irradiated sample did not change and thus remained to be a silicone, as analyzed by X-ray photoelectron spectroscopy. As a result, microswelling structures of approximately 1.3 µm height and 1.3 µm diameter were successfully obtained. The contact angles of water on the microstructured silicone were measured to be 150° and larger, clearly indicating superhydrophobicity. The mechanism by which the microswellings form their shape was discussed on the basis of the changes in the focal point and spot size during laser irradiation through the SiO2 microsphere.
This paper analyses the chemical structure and the amount of migrating polymer components into pollution layers on silicone rubber housing materials used in high voltage outdoor insulation and their respective effectivity in rendering the pollution layer surface hydrophobic. With Fourier transform infrared spectroscopy (FTIR) it was possible to observe a significant increase in the overall polydimethylsiloxane (PDMS) content of pollution layers on silicone rubbers between transfer times of weeks and several months. Contrarily the gas-chromatographic (GC) investigations revealed no significant increase during that period. Therefore, the low molecular weight (LMW) PDMS content detectable with gas chromatography - mass spectrometry (GCMS), which is generally accepted to be decisive for the transfer of hydrophobicity is likely only of subordinate importance. The LMW-PDMS extracted pollution was investigated again with FTIR, revealing that the LMWs detectable by GC constitute only a minor share of the PDMS transferred into the pollution layer. High-performance liquid chromatography (HPLC) allowed to directly corroborate this conclusion. With mass spectroscopy attached, the HPLC investigations also showed distinct differences in the molecular weight distribution of the transferred PDMS species for the investigated insulating materials. Accordingly, material specific PDMS content with individual molecular weight distributions seem to explain the observed differences in transfer capability of the investigated insulating materials.
We have investigated the relation between the molecular weight distribution (MWD) of diffusible species and the recovery rate of hydrophobicity of room-temperature vulcanizing (RTV) silicone rubber (SIR) using solvent extraction at various temperatures. Using gel permeation chromatography (GPC), we confirmed that the extract had a MWD ranging from a few hundreds to hundreds of thousands, and the average molecular weight (MW) of the extract having MW over 3000 increased with the increase of extraction temperature. By measuring the infrared (IR) absorption of siloxane migrating to the extracted SIR surface through a thin carbon coating, the aspect of migration of diffusible species was observed as a real time plot, and the time constant of the migration was calculated. According to the time dependence of IR-absorbance, the time constant was increased with the increase of MW of the extracts. It was found that the recovery rate of hydrophobicity of SIR was related to the MWD of the diffusible species.
This paper deals with recovery behavior of hydrophobicity of silicone rubber for polymer insulator housing after depositing artificial contaminant on its surface. Artificial contaminant was prepared based on TONOKO and sodium chloride. Recovery behavior of hydrophobicity of a specimen deposited with artificial contaminant was evaluated by receding contact angle and diameter of water droplets dropped on its surface. The specimen kept under high temperature and dry conditions recovered hydrophobicity faster than the specimen kept under low temperature and high humidity conditions. Also the specimen which contains much more amount of LMW (Low Molecular Weight) component recovered hydrophobicity faster than the specimen which contains less amount of LMW component. But adding low viscosity silicone oil to the silicone rubber lowered rubber properties, such as hardness, elongation, and tensile strength, and it was also clarified that an excess amount of low viscosity silicone oil made lowered tracking resistance performance of the specimen.
Dynamic contact-angle measurements based on Wilhelmy method were performed to evaluate time variation of loss and recovery of hydrophobicity of silicone rubber. The technique can determine both advancing and receding con-tact-angles of a plate sample and evaluate very short time variation of the surface properties.
By changing dipping speed of the samples to the water, immersing time and drying time in the air, some aspects of overturn and/or re-orientation of polar groups at the surface were observed as a change of the contact-angle. Also, the contact-angle of silicone rubber was compared with that of fluorinated silicone.
There are three principal approaches to control biofouling: (1) mechanical detachment of biofoulers if possible; (2) killing or inactivation of biofouling organisms using antibiotics, biocides, cleaning chemicals, etc. and (3) surface modification turning the substrate material into a low-fouling or non-sticking (non-adhesive) one. Such modification usually alters the surface chemical composition and morphology, surface topography and roughness, the hydrophilic/hydrophobic balance, as well as the surface energy and polarity.
In marine applications especially, current non-toxic biofouling control strategies are based mainly on the third approach, i.e., on the idea of creating low-fouling or non-adhesive material surfaces, an approach that includes development of strongly hydrophilic “water-like” bioinert materials. Strongly hydrophobic low-energy surfaces are preferable in industrial and marine biofouling control because of their relative stability in aqueous media and reduced interactions with living cells.
This chapter presents a brief overview of some possibilities for biofouling control by surface engineering. A number of related ideas will be discussed in this chapter, including: (1) the use of protein adsorption as a mediator of bioadhesion and biofouling, (2) physicochemical parameters influencing these phenomena, (3) theoretical aspects of cell/surface interactions, (4) some popular surface modification techniques, and (5) examples of successful biofouling control approaches.
Silicone rubber (SIR) shows superior performance when used outdoors, but its surface can be transformed from inherently hydrophobic to hydrophilic by the adsorption of contaminants. Al(OH)3, Al2O3, quartz powder and active carbon were selected as authentic contaminants. Hydrophobicity of the surface was determined using contact angle measurement. The results indicate that the adsorbability of the contaminants can strongly affect the hydrophobicity of contaminated SIR surface. The increasing rate of contact angle of specimens contaminated by Al(OH)3 was much faster than that by Al2O3 and quartz due to the adsorption of migrated low molecular weight (LMW) polydimethylsiloxanes. Specimens contaminated by active carbon could achieve surface hydrophobicity within 15 min because active carbon has high adsorbability. Surfaces of contaminated ultrapure SIR, polytetrafluoroethylene (PTFE) and glass remain hydrophilic because they contain no mobile LMW components. The addition of oligomeric polydimethylsiloxanes has little effect on the hydrophobicity of contaminants covered on SIR surface.
The fog water in the coastal area and in urban industrial area usually has high conductivity. It will cause rapid contamination accumulation on the composite insulators in service and affect their hydrophobicity state. This paper discusses about the change of hydrophobicity of the polluted silicone rubber surface in high conductivity fog environment. Ultrasonic fog generator was used to generate high conductivity fog and polluted silicone rubber was affected with damp in the mini fog chamber. The hydrophobicity status of the pollution surface was measured with the static contact angle method before and after staying in the damp. The results show that the hydrophobicity of polluted surface decreases after staying in the damp of high conductivity fog. The non-soluble deposit plays a role in the recovery of hydrophobicity of the pollution layer after affected with damp. It is more unfavorable for the recovery process with the higher ratio of salt deposit density to non-soluble deposit density, as well as the higher ambient humidity. Analysis shows that wetting in the damp of high conductivity fog will increase the salt deposit density of the pollution surface and enhance its ability of moisture absorption, causing the decrease of hydrophobicity of the polluted surface.
In order to provide the reference for the selection of the composite insulator and the experimental standard for the artificial pollution test, we investigated the influence of the environmental temperature on the flashover voltage of the composite insulator. The composite insulator with the alternating large and small shed design and the high temperature vulcanization silicon rubber specimen were used. Experimental results indicate that the diameter of the water wet area decreases with the increase of the migration time or the environmental temperature. The influence of the environmental temperature on flashover voltage is composed of the influence of the migration of the hydrophobicity and the influence of the different temperature out and in the fog chamber. The influence of the different temperature out and in the fog chamber on flashover voltage is slightly more remarkable(between 0.92%/°C~1.25%/°C) than that of the migration of the hydrophobicity. The artificial pollution test should be carried out with the environmental temperature between 15°C and 25°C, because that the two factors have a remarkable effect on the flashover voltage of composite insulator.
The insulation performance of room temperature vulcanized (RTV) silicone rubber coating will be impaired due to the heavy pollutions during its operation, thus the RTV coating is not widely used in some areas yet. Consequently, it is necessary to prepare anti-pollution-flashover hard coating with good self-cleaning ability. The hard coating was prepared using silicone resin which was added by a certain weight of molecular polydimethylsiloxane (PDMS) by chemical blocking. Self-cleaning was tested by small glass ball sliding down from inclined surface, multiple hydrophobicity transfer tests, inclined-planed tracking tests using coating specimens, and flashover experiments. The experimental results show that the hard coating has a hardness of Shore D30 and is more favorable for small solid particles sliding down from its surface than RTV coating; the hard coating has steady hydrophobicity transfer property, and its resistance to tracking and erosion achieves TMA2.5 rating, which is better than RTV coating; the pollution flashover voltage of the hard coating is no less than that of the RTV coating. Therefore, compared with the RTV coating, the hard coating can significantly reduce the accumulation of pollution on insulator surfaces, resisting tracking and erosion, and effectively preventing the pollution flashover.
There are many instances in polymer research and problem solving wherein GC/MS can be useful. The isolation and identification of compounds that relate to or are the cause of a problem or failure can be accomplished readily provided that they are sufficiently volatile to pass through a GC column. The apparatus described herein, Direct Dynamic Headspace Accessory, has been found advantageous as a means of releasing volatiles from samples directly in the GC injection port. Chromatograms of good resolution were obtained, enhancing the chances of making identification by MS. The temperature limit for devolatilization is that of the GC injection port, ca. 350°C in this case. In many instances the method is the best and most direct way of proving if an unusual or unacceptable condition developed from an external service condition or is related to the composition as manufactured.
Polydimethylsiloxane elastomer immersed in DC 200 silicone oil at temperatures of 180-200 degree C absorbs oil by two concurrent processes, a rapid and reversible diffusion process and a slower, irreversible process resulting from the chemical degradation of the sample. Absorption by diffusion reaches equilibrium in 200 h, and the absorption capacity depends on the amount of curing agent used in the preparation of the elastomer composite. The oil absorbed during this period can be removed by extraction. The absorption capacity is less for the elastomer composites containing silica and decreases linearly with the volume fraction of elastomer in the sample, indicating that only the elastomer absorbs oil. The rate of absorption depends on t**1**/**2 and has a weak temperature dependence. The diffusion coefficients are in the range 10** minus **1**2m**2/s and do not seem to be affected by the presence of filler. While the diffusion of oil does not appear to produce chemical changes in the elastomer, the elastomer is gradually destroyed by the slower chemical process, which promotes the absorption of additional amounts of oil.
Spontaneous adherence between two unvulcanized elastomers is the basis of tack, a property widely exploited in many technological operations. In an earlier study [1] on rubber-to-rubber adhesion we reported the tack strength of various rubbers and the effect of compounding and testing variables on the tack strength by a modified peel test. The first step involved in tack bond formation is the interdiffusion of rubber macromolecules. The measurement of diffusion has been reported by Skewis [2] and Bueche [3]. Recently Hamed [4] reviewed the three most fundamental criteria of tack of elastomers. Bussemaker [5] suggested that the green strength increased the tack strength of elastomers. This study was an attempt to measure the autohesive and adhesive tack of silicone and EPDM rubbers under various experimental conditions, and to identify and characterize the interaction and diffusion. Despite considerable effort, it appears from the literature that detailed information is still lacking concerning the diffusing rubber molecules on the contact surface and immediate subsurface regions. Clark and coworkers [6-9] reported X-ray photoelectron spectroscopic (XPS) data on the structure of polymers. In this letter it is intended to understand the diffusion phenomenon of one rubber into another from a knowledge of the depth of penetration. XPS and energy-dispersive X-ray (EDX) analysis techniques were employed. The elastomers used were EPDM (Mitsui EPT-3045, Mooney viscosity ML4 at 100°C 45, My = 8.9 × 104 g (gmol) -1 supplied by Mitsui Petrochemicals Corporation, Japan) and silicone rubber (NPC-80, Mooney viscosity ML 4 at 100 °C 80, My = 105 g (gmol) -1, supplied by Dow Corning Corporation, USA), bromobutyl rubber (Polysar bromobutyl X2, Mooney viscosity ML 4 at 100 °C 37.5, Mv = 7.91 × 105 g (gmol) -1, supplied by Polysar, Canada). For the determination of tack strength, sheets of 2.5 + 0.5 mm thickness were prepared in an electrically heated hydraulic press. One side of the sheet was backed by cloth. Sheeting was done at 100 °C for 5 min between two thin aluminium sheets. From these sheets, a narrow strip (20 mm × 120 mm) was allowed to come into contact quickly over another narrow strip of same size for a length of 100 ram. A dead load of 1 kg was applied in each case. The time and temperature of contact were the varying parameters. After the desired contact time, the average force required to separate the two strips was measured by peel test in a Zwick UTM 1445 at a
Elastomers based on polydimethylsiloxane (PDMS) are used as insulating material in outdoor electrical power applications. It is believed that migration of small molecule PDMS species plays an important role in the recovery of hydrophobicity of oxidized or polluted PDMS elastomer surfaces. This paper reports data on diffusivity and solubility of low molar mass PDMS liquids in PDMS rubbers (8000 < Mc < 16,000 g/mol) obtained by sorption measurements. It was found that the diffusivity (D) of linear PDMS liquids was approximately independent of the concentration of penetrant and that in the molar mass range 400 < Mc < 18,000 g/mol it decreased with molar mass (Mc) of the diffusing liquid according to D Mc−0.8. Theory and previous data for other oligomers and elastomers predict that D is proportional to M−1. Linear PDMS liquids of lower molar mass exhibited a stronger molar mass dependence. The diffusivity of a given PDMS liquid increased with increasing elastomer crosslink density. The activation energy of the diffusivity was constant at 15.5 ± 2 kJ/mol for linear PDMS liquids of Mc larger than 1000 g/mol−1 with only a negligible influence of network density and filler content. The activation energy of the lowest molar mass penetrant was considerably lower, 6 to 7 kJ/mol. The solubility increased markedly with decreasing molar mass of the penetrant and with decreasing elastomer crosslink density.
The present contribution describes an analytical technique which
can be used to investigate degradation phenomena of silicones and
provide a quantitative evaluation of surface degradation. The technique
employed in the present investigation is based on the use of gas
chromatography/mass spectrometer (GC/MS) methods
Results of a study performed to obtain a better understanding of
the material characteristics responsible for hydrophobicity recovery
leading to a high wet surface resistance in silicone rubbers used for
outdoor HV insulation, are reported. The samples were obtained from new
and artificially aged HV insulators using HTV silicone rubbers (3
different formulations) as weathershed and RTV silicone rubbers (2
different formulations) as a protective coating. The main experimental
facilities employed consist of a salt fog chamber for artificially aging
the insulators, and a scanning electron microscope (SEM) for material
studies. New results of practical significance that have emerged from
this study are: (1) hydrophobicity recovery, predominantly due to
diffusion of low molecular weight (LMW) silicone polymer chains, occurs
with only a fraction (<20%) of the total LMW polymer content
initially available in an unaged material surface, (2) LMW chain
regeneration and hence surface hydrophobicity recovery occurs even after
the initial supply of LMW polymer is depleted, and (3) hydrophobicity
recovery is significantly affected by ambient temperature. The results
show the same pattern for different formulations of HTV and RTV rubber
materials studied. The X-ray Mapping feature in the SEM provides a
visual indication of the diffusion process which is a noteworthy
contribution
Studies are reported on decay and recovery of hydrophobic
properties on clean and contaminated surfaces of silicone rubber based
RTV coating. Treatment that led to hydrophobicity losses were water
immersion and water condensation with and without a presence of dc
electric field. Contact angles (receding angles) and surface leakage
currents were measured on the treated surfaces. A suppression of the
leakage current was observed and its link to the diffusion of the low
molecular weight fraction from the polymer bulk and dry band arcing is
discussed. Attempts aiming to restore the lost hydrophobic properties
are also described. As a means of recovery, resting at ambient
atmosphere, under UV irradiation, and in vacuum at elevated temperature,
was applied. The efficiencies were verified through measurements of
receding angles, surface leakage currents, and thermally stimulated
depolarization currents
An RTV silicone rubber coating applied to insulators in a 345 kV
switchyard located near Long Island Sound has depolymerized and become
putty-like after six years of service. Coating samples were taken from
high creep insulators which were energized and weathered, weathered but
not energized, and neither weathered or energized (warehouse storage).
The unweathered and unenergized coating did not appear to degrade. The
weathered but unenergized coating had degraded measurably but was still
in an operational state. The energized and weathered coating has become
putty-line and lost all physical integrity. This suggests aging by a
combination of chemical, photochemical (weathering), and electrochemical
mechanisms
Room temperature vulcanizing (RTV) silicone rubber coating is
increasingly being used by power utilities to overcome the contamination
problems encountered in ceramic and glass insulators under wet
conditions. The formulation of the RTV coating system is currently
undergoing an intensive development in an effort to optimize its
performance and in particular to extend its lifetime. While there are
many forms of RTV, those used for insulator coating purposes invariably
use a base of polydimethylsiloxane. The amount of RTV used in the
service application is also important as it impacts on both the cost of
the protection offered and on the electrical performance. This paper
reports on a study of the influence of the coating thickness, different
substrates, addition of silicone fluid to the coating formulation and
different carrier solvents on the leakage current, pulse current count
rates and the lifetime of the RTV coatings. It has been found that all
these parameters affect the electrical performance under wet and
contamination condition in a salt-fog. The optimum coating thickness has
been determined for a fixed substrate. Mechanisms by which these four
parameters affect the performance are suggested and discussed
In a long-term outdoor test with high direct and alternating
voltages, silicone and ethylene propylene diene monomer (EPDM) rubber
composite insulators have, at the beginning, shown a performance
superior to that of glass and porcelain insulators. In the long-term
test, however, the silicone rubber composite insulator has, in spite of
the aging of both insulator types, kept its good performance, while the
performance of the EPDM rubber composite insulator was drastically
deteriorated. In order to get a better insight into the results
obtained, the wettability and the surface structural changes of the
insulators were studied by the drop deposition method using a goniometer
and by advanced techniques such as scanning electron microscopy,
electron spectroscopy for chemical analysis, Fourier-transform infrared
spectroscopy, and secondary ion mass spectrometry. The results show that
the differences in performance are related to the differences in the
surface structural changes and in the dynamic ability of the surface to
compensate the aging. Silicone rubber insulators have higher repellency
than the EPDM insulators, especially when aged. The reason seems to be
low silicone polymer diffusion from the bulk which covers the insulator
surface, embeds the pollutants, and keeps the surface hydrophobic.
Porcelain insulators with silicone elastomer coatings show lower water
repellency than insulators with massive silicone rubber sheds. When
aged, EPDM insulators do not, however, seen to possess the same dynamic
recovery of their surface, thus becoming hydrophilic
The results of a study on silicone rubber and ethylene propylene
diene monomer (EPDM) polymers are presented. The study was done to
understand the mechanisms involved in the loss and subsequent recovery
of surface hydrophobicity due to dry-band arcing and to investigate
various experimental techniques which could be used for characterizing
surface hydrophobicity. The materials are subjected to dry-band arcing
in a log chamber. Several techniques such as measurement of contact
angle, determining surface composition using electron spectroscopy for
chemical analysis (ESCA), and measuring crossover voltage (COV) using a
scanning electron microscope are examined. The experimental results and
theoretical calculations demonstrate that the mobile species in the
polymer are responsible for the surface hydrophobicity. In terms of
repeatability and simplicity of measurement, the COV determination
appears to be the most suitable technique for hydrophobicity studies
The results of a study on the suppression of leakage current on
room temperature vulcanizing (RTV) silicone rubber coated porcelain
suspension insulators in a salt-fog chamber are reported. A comparative
study of the performance of coatings on different parts of suspension
insulators with RTV is reported. The results of attenuated total
reflection (ATR) Fourier transform infrared (FTIR) spectroscopy are
reported. On the basis of these results, a suppression mechanism for
leakage current is suggested. A mechanism for the suppression of leakage
current on RTV coated porcelain and silicone rubber insulators dealing
with the temporary loss of hydrophobicity and its return is suggested
The wettability of aged surfaces and of the bulk of naturally aged
silicone and EPDM (ethylene-propylene diener monomer) insulator housings
and silicone elastomer insulator coatings was studied. The samples were
taken either directly from the insulators or treated by exposing them to
corona discharges and/or to saline pollution. The results show that the
contact angles of the silicone rubber insulator surfaces are larger than
the contact angles of the RTV (room-temperature vulcanizing) silicone
rubber coating and of the EPDM rubber insulator surfaces, especially
when the surfaces are aged. When the insulators were exposed to corona
discharges, the contact angles of the silicone rubber insulators are
reduced but after exposure recover with time. The contact angles of the
EPDM rubber insulators, however, continue to reduce after the exposure.
When exposed to artificial saline pollution, the silicone rubber
insulators show a limited recovery of their contact angles with time,
while when exposed to corona discharge after saline pollution, they show
a recovery of the contact angle after the exposure. The recovery time is
dependent on the exposure time to the corona discharges. The EPDM
samples do not show any recovery of their contact angles, either when
left to rest after the salt deposition or, when after the salt
deposition, they undergo a subsequent exposure to corona discharges.
Furthermore, such an exposure may be deleterious for the EPDM polymer
and the insulator surface may become completely hydrophylic
The results of a study on the loss and recovery of hydrophobicity
of RTV (room-temperature vulcanizing) silicone-rubber insulator coatings
in a salt-fog chamber are reported. The results complement those
previously reported on the ability of the coatings to suppress leakage
current and insulator flashover. The temporary loss of hydrophobicity
caused by dry-band arcing and the subsequent recovery are studied in
depth. The gradual loss of hydrophobicity as determined from the leakage
current and the contact-angle measurements is shown to be related to the
physical changes to the coating brought about by dry-band arcing
Measurements and analysis of contaminant samples from a nuclear
power station which has suffered flashovers due to wind-swept salt spray
are presented. The results of a program of laboratory tests on
artificially contaminated porcelain station insulators, with and without
a room temperature vulcanized (RTV) silicone rubber coating, are
reported. Data from clean fog and salt mist tests are used to determine
the improvement in flashover voltage level over a range of high
contamination severity on RTV-coated insulators. Visual observations and
leakage current measurements made during surface discharge activity are
utilized to demonstrate the influence of the RTV coating on discharge
behavior. Switching impulse test results are also presented
The ability of room temperature vulcanizing (RTV) silicone rubber
insulator coatings to suppress leakage current, thereby preventing power
outages due to contamination, was studied. The RTV coatings were applied
to porcelain suspension and line post insulators for a comparative study
of the performance of coated and uncoated insulators. A study of the
role of alumina trihydrate (ATH) which is used to impart tracking and
erosion resistance to RTV coating is reported. A study of the diffusion
of silicone fluid from the bulk to the surface of the RTV silicone
rubber having various ATH filler levels using attenuated total
reflection (ATR) Fourier transform infrared (FTIR) spectroscopy is also
reported. It has been found that the diffusion rate of the silicone
fluid from the bulk to the surface of the coating after extensive dry
band arcing decreases with increasing filler level