Analyzing the effects of thermal stress on insulator papers by solid-state 13C NMR spectroscopy

Abstract

Million tons of cellulosic paper have been used for insulating coils in oil-filled electrical power transformers, thereby assuring the electricity supply for our societies. The high working temperatures in transformers constantly degrade paper insulators throughout their service life of up to 40 years. We approached the structural changes in oil-immersed cellulosic paper samples upon thermal stress in a study that compared unbleached softwood Kraft paper used as insulator paper with pure cotton cellulose paper. The model experiments used a thermal treatment in transformer oil at 170 °C for up to 14 days. The samples were characterized by means of 13C CP/MAS NMR spectroscopy, mainly based on deconvolution of the C4 resonance. An automated, fast, and reproducible C4 resonance deconvolution employing the “Peak Analyzer” tool of OriginPro 2020 (OriginLab Corporation, USA) was developed and used to exploit 13C CP/MAS NMR spectroscopy for the characterization of thermally stressed paper samples. Our results show that thermally induced structural changes depend heavily on the composition of paper, that hornification and coalescence of fibrils take place, and that the allomorph composition of cellulose crystallites is altered under the given conditions. Graphical abstract

Impact of residual extractives on the thermal stability of softwood Kraft pulp

“…These findings suggest that the arabinoxylan fraction of softwood Kraft pulp is thermally less stable than the galactoglucomannan fraction. This is in accordance with recently published results obtained by the analysis of paper strips made of Kraft pulp aged in transformer oil at 170 °C (Jusner et al 2022a(Jusner et al , 2022b. In general, the presence of extractives facilitates the pyrolytic decomposition of the hemicellulose fraction in softwood Kraft pulps.…”

“…2b for the respective isothermal TGA curves *Only rather noisy solid-state 13 C CP/MAS NMR spectra could be obtained from samples subjected to isothermal TGA in a N 2 /O 2 atmosphere, which complicated reproducible C4 signal deconvolution a Derived from two ATR FTIR spectra per sample (the relevant bands are indicated in Fig. 6b), b Derived from solid state 13 C NMR spectra via C4 curve fitting, c Calculated with formula (8) of Newman (1999) inserting CI and the average spacing between cellulose chains derived from the 24-chain model suggested by Oehme et al (2015) as described previously (Jusner et al 2022a) sample are compared before isothermal TGA. Only some minute differences regarding the glucose content were detected.…”

“…The raw data was processed with the Bruker Top-Spin 4.0.7. software (Bruker, Germany) and the C4 (91-79 ppm) deconvolution was carried out with the Dmfit 19.12.11 software (CEMHTI CNRS UPR3079, France) according to Wickholm et al (1998) and Zuckerstätter et al (2013). Crystallinity indices (CI) and lateral crystallite dimensions were obtained as described by Jusner et al (2022a).…”

“…The motivation for this study was to explore whether these extractive residues, and extractives in general, also affect the thermal stability of pulp fibers, in connection to our previous studies on the thermal stability of papers in power transformers and the analysis of the thermal effects (Jusner et al 2021(Jusner et al , 2022a(Jusner et al , 2022b. It has been proposed that extractives found in wood are likely to have a deteriorating effect on the thermal stability of wood fibers (Shebani et al 2008(Shebani et al , 2009Poletto et al 2012;Chen et al 2014;Poletto 2016;Hamada et al 2017).…”

Assessment of Electrothermal Pretreatment of Rambutan (Nephelium lappaceum L.) Peels for Producing Cellulose Fibers

“…The broad halo in the RP is an indication of amorphous components that are removed by ohmic heating and bleaching. The CI decrease in OH-60B6 is due to the prolonged pretreatment of fibers causing thermal degradation of the cellulose crystals and reduction of the crystallite sizes . This is in full agreement with the results from the whiteness experiments discussed above.…”