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

A method for lignin quantification is presented that is based on densitometry and multivariate calibration by partial least squares (PLS). Densitometry removes interferences otherwise caused by undissolved liquor components and solvents, while multivariate calibration accounts for interferences by other sample components, such as extractives or chromophoric compounds. Method validation predicted a bias very close to 0% and an uncertainty of 10% (kraft) to 20% and less (lignosulfonate). In trials with three industrial kraft liquors, biases from 0 to 5% were observed; for a tested lignosulfonate, a bias of 6% was obtained. By combining this method with thin-layer chromatography, other liquor components can be determined simultaneously with the lignin content, for example monosaccharides in sulfite liquors (total time per sample: 13 min). If only the lignin content is of interest, chromatography is unnecessary, and more than 200 samples can be applied to the plate in a dense pattern (time per sample: 2 min). Compared to other methods, this measurement is fast, gives correct results, is inert toward matrix components, and can be coupled with a chromatographic analysis. Its only prerequisite is the isolation of a suitable reference lignin to be used as an external standard.