Condensation curing silicone sealants and adhesives are materials that are used to connect substrates to each other in various industries, especially where harsh conditions are limiting the use of ...organic materials. Chemistry of condensation curing silicones is reviewed in this publication through an historical perspective, which provides an insight into the challenges faced by the early formulators. The impact of the formulation ingredients can be very significant on the uncured, curing and/or cured properties of the material. Their main influence is described with a special focus on the main curing systems, i.e., acetoxy, alkoxy, oxime, amine, acetone, and amide. Cure characteristics are summarized and compared across their chemistries. Alkyl tin carboxylates and alkoxy titanium catalysts are the most used condensation catalysts. They are acting through a different mechanism, which is compared briefly. Typical features and principles about one-part and two-part silicones are described and related to their physicochemical properties. Finally, a brief review of the latest developments and trends is depicted.
•In situ μ-CT imaging of RTV pyrolysis conducted for multiple heating rates up to 1000°C at synchrotron facility.•Porosity and pore size quantified for RTV during pyrolysis.•Volumetric shrinking and ...swelling measured on solid RTV in situ.•Scanning electron microscopy used to analyze the remaining nanoporosities present in each sample.•High heating rate revealed incomplete pyrolysis at char temperature
Quantitative microstructural analysis of Room Temperature Vulcanized (RTV) silicone pyrolysis at high temperatures is presented. RTV is used as a bonding agent in multiple industries, particularly filling gaps in ablative tiles for hypersonic (re-)entry vehicles and fire prevention. Decomposition of RTV is resolved in real time using in situ high-temperature X-ray computed micro-tomography. Tomographies are acquired every 45 seconds for four different linear heating rates ranging from 7 to 54°C/min. The microstructure is resolved below 5 μm/pixel, allowing for a quantitative analysis of the micro-structural evolution and porous network development. Results are heating rate dependent, and are evaluated for bulk sample volume change, porosity, pore network size, and densification from X-ray attenuation. The outcome of this work is critical to develop multi-physics models for thermal response.
Microstructure of pyrolyzing RTV silicone Oruganti, Sreevishnu; Mansour, Nagi N.; Panesi, Marco ...
Polymer degradation and stability,
January 2023, 2023-01-00, Volume:
207
Journal Article
Peer reviewed
Open access
•Mass loss, foaming and microstructure evolution of RTV as a function of temperature are discussed.•Pore size distribution and porosity of RTV were measured for charred RTV.•An intumescence model ...capturing all the observed multiphysical phenomena is proposed.
This works discusses the high temperature decomposition of Room Temperature Vulcanized (RTV) silicone, an adhesive material used as a gap filler in ablative heatshield for planetary atmospheric entry, and in fire-protection applications. The high temperature behavior of RTV is characterized by intumescence, shrinkage, foaming and formation of a glassy char. In heatshield applications, these phenomena lead to a mismatch between the ablative response of RTV and that of carbon-phenolic tiles, posing numerous design challenges such as tile layout selection, installation of heatshield sensors, prediction of laminar-to-turbulence transition and assessment of heatshield robustness. The ablation response of RTV is not modeled in current design tools because of the lack of data on its high temperature behavior. In this paper, thermogravimetric analysis was conducted to quantify the mass-loss of RTV during pyrolysis, as a function of heating rate. Samples were imaged using environmental scanning electron microscopy at high temperature to observe the microstructural evolution of RTV during charring. Helium pycnometry, mercury intrusion porosimetry and nitrogen adsorption techniques, combined with micro-tomography imaging, were used to quantify the pore size distribution and porosity of charred RTV. Results show that the porosity of charred RTV is about 82% and that the predominant pore sizes are 2, 23, 39 and 98 microns. The net volumetric swelling due to pyrolysis of the material was found to be 10% when heated to 1000 °C. Physical phenomena occurring during high temperature decomposition are discussed based on the experimental observations.
•The breakdown rule of coated inorganic insulators under impulse is illustrated.•Flashover processes under steep-front impulses are recorded.•The breakdown mechanism under steep-front impulse is ...demonstrated.•Optimization approaches for coating insulators are proposed.
Insulators play the vital role in the power transmission, including traditional porcelain/glass insulator and silicone rubber based polymeric insulator. By coating the glass/porcelain insulators with silicone rubber, the advantages of organic and inorganic dielectrics are integrated. However, it was found that the breakdown probability of coated insulators under steep-front impulse increases significantly. It suggests the possible degradation of insulator material and might become the hidden danger of power system. Therefore, in this research, a great deal of steep-front impulse voltage tests are carried out over various types of insulators. It demonstrates that the performance degradation is introduced by RTV coating instead of intrinsic defects. Then, based on the analysis of flashover paths and breakdown locations, the breakdown mechanism of coated insulator under steep-front impulse is illustrated. Also, numerical simulation of electric field distribution is performed for verification, taking the surface arc development into account. It indicates that the development of surface arc is distorted by the coating which results in the intense local field and dielectric breakdown. Next, different controlled experiments are designed and conducted to validate the theory. Besides, the corresponding modification towards the current coating method is proposed.
The present work involves the investigation of ionizing radiation effects on silica filled poly (dimethyl siloxane) foam vulcanized at room temperature. In order to better predict aging effects in ...these materials, it is important to understand the influence of irradiation on structural-rheological property relationships. Polysiloxane foams were subjected to moderate doses of gamma irradiation in an inert atmosphere and characterized by thermal (DSC and TGA), chemical (FT-IR, NMR, Mössbauer, mass spectroscopy, EPR, solvent swelling), microscopy (SEM and AFM), and mechanical (uniaxial compressive load) techniques. Radiation exposure induced cross-linking reactions that predominated over chain scission reactions for the dose range investigated. No long-lived radiation-induced radicals were detected and the porous structure of the irradiated foam remained unchanged. Radiation exposure resulted in gas evolution, decrease in crystallization levels, slight changes in chemistry, and decrease in the molecular weight between cross-links, thereby hardening the foam.
Room temperature vulcanised (RTV) silicone rubber is widely used in electrical power system, applied to the glass or porcelain insulator surface to enhance insulation. RTV coatings get contaminated ...after long-term operation and need to be recoated. Contaminants accumulate and embed into the surface of base RTV coating, forming a hard-to-remove inorganic transition layer. Such sandwiched contaminant layer destroys the adhesion between original and reapplied silicone rubber coating greatly. Authors’ study innovatively reports a method to enhance interfacial bond properties of the RTV recoating with sandwiched contaminant layer, by using an atmospheric-pulsed discharge plasma jet (APPJ) for surface treatment. Water boiling test and 180-degree peel test were conducted to evaluate interfacial bond properties macroscopically. Scanning electron microscopy (SEM) was applied to observe the cross-section of recoating interface, while Fourier-transform infrared spectroscopy (FTIR) to detect chemical changes of contaminant layer, giving out microscopic and theoretical explanation.
In this research, a novel high permittivity ceramic silicone composite substrate (HPCSCS) based antenna design is proposed for RF energy harvesting (RFEH). The HPCSCS is prepared using ...high-permittivity barium titanate (BaTiO3) ceramic powder mixed with RTV silicone sealant (RTVSS) which possesses a high permittivity value of 11.9. Five different antenna designs were simulated in CST software and one design has been finalized based on design geometry, bandwidth, gain, and reflection coefficient. Moreover, the proposed antenna design is optimized with the addition of a coplanar waveguide. The designed antenna has a thickness of 0.06 mm and provides a wide operating band range from 0.7 GHz to 3 GHz. It provides a reflection coefficient of –32 dB and a positive gain of 1.84 dB at the operating frequency of 2.66 GHz demonstrating the excellent wide bandwidth of 885 MHz. A prototype of the antenna is fabricated and mounted on an HPCSCS for measurement through a vector network analyzer with a good match between the simulated and measured results. The rectenna system (antenna + rectifier) for RFEH has been developed and tested, which achieved a maximum rectifier conversion efficiency of 57 % at 0 dBm and produced a maximum output voltage of 1.35 V at 0 dBm.
•The damage of location and lateral length have little influence on the flashover.•The damage of longitudinal length was the major factor on the flashover.•The “effective path” can be used to easily ...analyse the effect of multiple damages.•The flashover voltage was in a linear relationship with the “effective path”.
Under conditions of high temperature, high humidity, and strong ultraviolet radiation, RTV coating on the insulators may get aged, powdered and even peeled off. The physical damage of the RTV coating on the insulator surface can result in a reduction in the pollution flashover performance. Therefore, this study investigated the influence of location, lateral length, and longitudinal length of the damage on the pollution flashover voltage of RTV-coated insulators. From the experimental results, it was found that the longitudinal length of the damage along the creepage distance direction was the most influential factor on the pollution flashover voltage. According to the flashover voltage function obtained by fitting, the concept of “effective path” distance was proposed. This concept can be used to analyse the influence of multiple damages on the pollution flashover voltage. To validate the analysis, verification experiments were performed using typical industrial insulator. The results showed that the pollution flashover voltage of the insulator was also in a linear relationship with the proposed “effective path” distance, which confirmed the mechanism analysis.
Poor quality of room-temperature vulcanized silicone rubber (RTV-SIR) coating on insulators, such as damages, inclusions, debonding, uneven or substandard thickness, and so on, seriously deteriorates ...the external insulation performance and brings potential threats to the safety of power grids. This article proposed a convenient, quantitative, and nondestructive inspection method for room-temperature vulcanized (RTV) coating using multimode features of active infrared thermography (AIT). First, the thermal wave model of RTV coating was established to explain the theoretical basis for defect detection and thickness measurement of RTV coating based on the surface transient temperature field. Typical defects of RTV coating were detected, and the defect identification was enhanced by the statistical reconstructed thermal images. The frequency-domain features and the reciprocal square root of the minimum phase frequency (RSRMPF), which was proved to have a linear relationship with the coating thickness, were extracted from the phase spectrum of transient thermal response to quantitatively characterize the micrometer thickness of RTV coating. Particularly, the frequency aliasing phenomenon in small thickness measurement when the sampling frequency is not high enough was reported. And the genetic algorithm was adopted to implement the transient thermal response fitting to overcome the distortion of the phase spectrum induced by the frequency aliasing to obtain the accurate fitted linear calibration line. It has been demonstrated that AIT would have promising application prospects in industrial-quality inspection of RTV coating.
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