Experimental and molecular level analysis of natural ester delaying degradation of cellulose insulation polymer

Abstract

Developing green insulation materials has become an important direction for future research. As a new environmental insulation liquid, natural ester can effectively delay cellulose insulation polymer ageing. Mastering the mechanism of natural ester delaying the cellulose ageing could promote its application in power transformers. In this study, based on the accelerated thermal ageing of pressboard immersed in natural ester and 25# mineral oil at 120°C for 221 days, the thermal ageing rate and the levelling off degree of polymerisation (LODP) for two kinds of oil-impregnated pressboards were compared. The molecular level mechanism of natural ester delaying cellulose degradation and keeping higher LODP value was proposed using molecular dynamics simulation. At the early stage of thermal ageing, cellulose with a higher degree of polymerisation (DP) can form many hydrogen bonds with hydronium ions, and their binding energy is stronger, which can accelerate the acid hydrolysis of cellulose, causing the DP of pressboard immersed in natural ester and mineral oil to decrease rapidly. With the breaking of the cellulose molecular chain, the number of hydrogen bonds and binding energy between cellulose and hydronium ions becomes smaller and the thermal ageing rate decreases gradually. At the end stage of ageing, it is difficult for the hydronium ions to enter the crystalline cellulose, acid hydrolysis of crystalline cellulose is hard to conduct, leading to the occurrence of the LODP phenomenon. The degradation rate of cellulose in natural ester is only 50% of that in mineral oil. The cellulose LODP value in natural ester is over 200 larger than that in mineral oil. Compared with the mineral oil-cellulose system, natural ester can form hydrogen bonds with hydronium ions and lead to fewer hydronium ions in cellulose, fewer hydrogen bonds and smaller binding energy between cellulose and hydronium ions, which can reduce the acid hydrolysis reaction of cellulose. This study provides a theoretical explanation for natural esters to delay the ageing of insulation paper.

Degree of polymerisation estimation for insulation paper immersed in silicone oil using frequency domain dielectric spectroscopy

“…In the ageing analysis and evaluation of oil‐paper insulation, the zero‐order ageing dynamic model proposed by scholar Ekenstam is widely used in the ageing data analysis of oil‐paper insulation. The expression of the model is as follows [4]: $$$\frac{1}{D{P}_{\mathrm{t}}}-\frac{1}{D{P}_{0}}=kt$$$ where DP t is the DP of insulation paper after ageing time t , DP 0 is the initial DP of insulation paper, k is the ageing reaction rate, and the calculation formula of k , namely Arrhenius equation, and its expression is as follows: $$$k=A{\mathrm{e}}^{-\tfrac{{E}_{\text{av}}}{RT}}$$$ where A is the pre exponential factor, and the zero‐order ageing kinetic model considers it to be a constant quantity, so it can be considered as an invariant pre exponential factor. E av is the average activation energy of chemical reaction.…”

“…In the ageing analysis and evaluation of oil-paper insulation, the zero-order ageing dynamic model proposed by scholar Ekenstam is widely used in the ageing data analysis of oilpaper insulation. The expression of the model is as follows [4]:…”

“…Degree of polymerisation (DP) and tensile strength are the main ageing indexes of oil-paper insulation, among which DP is most widely used, and it is the most intuitive and recognized target to reflect the ageing state of insulation paper [3][4][5]. Many and the most widely used zero-order ageing kinetic model was proposed.…”

Investigation of Polymer Aging Mechanisms Using Molecular Simulations: A Review

“…Zheng et al [96] analyzed and tracked CO 2 and CO by RMD simulation; these reaction pathways were given a lot of attention because the CO 2 /CO ratio could reflect the aging state of cellulosic insulating paper. Hao et al [97] simulated the internal aging mechanism of natural ester delaying the aging of the cellulose insulation polymer in their MD simulation, using the polymer consistent force field (PCFF). They found that natural ester could form hydrogen bonds with hydronium ions, which led to fewer hydronium ions in the cellulose and finally reduced the acid hydrolysis reaction of cellulose.…”

Mechanism study of the conductivity characteristics of cellulose electrical insulation influenced by moisture

“…Instead, a combination of molecular dynamics (MD) simulations with quantum chemistry (QC) calculations can provide such microscopic insight. [22][23][24] MD allows a detailed study of hydrogen bond characteristics at the molecular scale, 25 so it is a powerful tool for exploring microscopic moisture adsorption characteristics. Due to the disordered nature, the charge transport in cellulose is likely of the hopping type between trap localized states.…”