Accelerated ageing of wood Kraft insulating paper by means of a pulsed KrF excimer laser radiation

We report on the successful use of a KrF excimer laser as a highly promising tool to investigate the accelerated ageing of wood Kraft papers used as insulating components in high-voltage power transformers. The effects of varying both the laser repetition rate (5 to 180 Hz) and the laser energy density (0.02 to 0.11 Jcm^-2) on the degradation of wood Kraft paper in air were systematically investigated. During the laser irradiation experiments, the temperature of the paper samples was real-time measured by means of an IR camera. It is shown that the ageing temperature of the paper can be controlled very precisely by simply adjusting the laser parameters. Indeed, levelling-off temperatures in the 25–320 °C range can be reached after only 3 s of laser exposure, by either varying the repetition rate at a fixed energy density or changing the laser fluence at a given pulse rate. By performing both scanning electron microscopy (SEM) observations and measurements of the degree of polymerization (DP_v) of the laser irradiated paper samples, we were able to demonstrate that the laser irradiation induces a very rapid degradation of the paper samples (typical laser exposure times are around 35 s to be compared with 10 days for standard dynamic heating based ageing tests). Both the DP_v of the irradiated paper samples and the average width of the corresponding cellulose fibers as measured from SEM micrographs, are shown to reduce by about 50% following their laser heating at 320 °C for 35 s of exposure time. It is pointed out that the observed laser-induced degradation is predominantly a photothermal process. Finally, by using gas chromatography/mass spectrometry (GC/MS), we were able to show that low molecular weight compounds such as acetaldehyde, methyl formate, acetone, and methanol are produced not only by the laser ageing process but also in real on-field transformers that underwent a severe short-circuit. PACS 42.62.-b; 61.80.Ba; 83.80.Mc; 84.70.+p