Micro characterization and degradation mechanism of liquid silicone rubber used for external insulation

Abstract

In recent years, some hollow core composite insulators, of which housings were made of liquid silicone rubber (LSR) of current and voltage transformers used in China Southern Power Grid have showed different levels of degradation and cracking phenomena. Based on SEM, FTIR, XPS and XRD methods, analyses have been conducted from several aspects such as organic functional groups concentration, microscopic appearance, element valence etc. It is found that organic groups (mainly Si-(CH 3 ) 2 ) on the side-chains of LSR are much more vulnerable than the backbone during the degradation process, thus Si-O/Si-C absorption peak ratio could be applied to characterize the degradation level of LSR. For severely aged LSR samples, the ratio is around 1.787-2.436. The valences of Si element and Si-O, Si-C functional group relative concentration of the aged LSR surface have changed and crystal structure is formed. Macro properties tests show that the surface of aged LSR is almost hydrophilic and tracking and erosion resistance is deteriorated remarkably. It is concluded that the crystallization caused by thermal/optical oxidation over crosslinking reaction is the main reason why degradation occurred.

Effect of ultraviolet‐A radiation on surface structure, thermal, and mechanical and electrical properties of liquid silicone rubber

“…Based on XPS and FTIR analysis, it was found that the content of Si(O) 3 and SiO increased after LSR was aged by UV‐A radiation. These indicate that the interactions between fillers and the matrix loosened, but that the degree of crosslinking of the matrix increased after UV‐A aging, which is consistent with the aging performance of LSR in a normal aging environment . The interphase between carbon fiber and reinforced polymer reduced during UV exposure due to the change in chemical crosslinking of the polymer near interphase region.…”

“…However, according to the UV aging mechanism of HTV silicone rubber, Si(O) 4 should increase after aging, which is contrary to the reported results. For silicone rubber materials, the surface becomes porous, and a large number of filler particles are exposed on the surface after UV aging . Therefore, the SiO 2 fillers on the surface were easily lost during the testing process, which resulted in reduced Si(O) 4 and Si and O content.…”

“…Based on XPS analysis, the loss of SiO 2 fillers led to a reduced content of Si. Although more fillers in the loose layer are easier to lose when the samples were broken during the preparation, the depth still corresponded to the loose layer that was generated during the UV‐A aging …”

“…The wavenumbers corresponding to the characteristic absorption peaks of LSR are depicted in Figure . The full FTIR spectrum can be found in the Supporting Information Figure S3.…”

“…The internal functional groups in silicone rubber can be analyzed by FTIR due to its detection depth from several micrometers to tens of micrometers . A Tensor 27 Spectrometer (Bruker Optics, Germany) was used to measure the characteristic peaks of silicone rubber in the spectral range from 4000 to 600 cm −1 .…”