DOI: 10.1007/s00339-005-3209-1 Appl. Phys. A 81, 57–63 (2005) Materials Science & Processing Applied Physics A j. jalbert 1,2, ✉ r. lacasse 2 m.a. el khakani 1 r. gilbert 2 Accelerated ageing of wood Kraft insulating paper by means of a pulsed KrF excimer laser radiation 1 Institut National de la Recherche Scientifique, INRS- ´ Energie, Mat´ eriaux et T´ el´ ecommunications, 1650, boulevard Lionel-Boulet, Varennes, Qu´ ebec, J3X 1S2 Canada 2 Institut de Recherche d’Hydro-Qu´ ebec (IREQ), 1800, boulevard Lionel-Boulet, Varennes, Qu´ ebec, J3X 1S1 Canada Received: 9 July 2004 /Accepted: 20 December 2004 Published online: 2 March 2005 • © Springer-Verlag 2005 ABSTRACT 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 dens- ity (0.02 to 0.11 J cm −2 ) on the degradation of wood Kraft paper in air were systematically investigated. During the laser irradi- ation 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. In- deed, 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 elec- tron 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 (typi- cal 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% follow- ing 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 acetalde- hyde, methyl formate, acetone, and methanol are produced not only by the laser ageing process but also in real on-field trans- formers that underwent a severe short-circuit. PACS 42.62.-b; 61.80.Ba; 83.80.Mc; 84.70.+p 1 Introduction Owing to their excellent insulating properties and low cost, wood Kraft papers have long been used by elec- trical utilities as the insulating material of choice in high- voltage (HV) power transformers. The main constituent is ✉ Fax: +1-450-652-8424, E-mail: jalbert.jocelyn@ireq.ca cellulose, a linear homopolymer composed of cyclic β-D- glucopyranosyl units jointed by C-1−O−C-4 ′ glycosidic bonds. Due to a combination of oxidative, hydrolytic, and thermal degradation mechanisms that affect all cellulose- based materials, the degradation of the insulating paper be- came a source of concern for utilities as equipment ages. As a result of this action, the average degree of polymerization of the cellulose (averaged number of cyclic β-D-glucopyranosyl units in the cellulose molecular chains) decreases asymptot- ically from an original value of about 1000 units to a lim- iting value in the region of 200. At this stage, paper with 20% of its original tensile strength is recognized as being at the end of its useful life [1]. In open-breathing HV power transformers which typically contain ∼ 11 tons of paper and pressboard, and up to 45 tons of oil, these papers in con- tact with an oxygen-saturated mineral oil may present up to about 2 (w/w) % of moisture and, depending upon opera- tional conditions, may be exposed to temperatures ranging from 60 to 110 ◦ C. Under such conditions, it is known that the prevailing degradation mechanisms involve the hydroly- sis of the molecular chains and the oxidation of the primary C-6 and secondary C-2 and C-3 hydroxyl groups of the β-D- glucopyranosyl units, both leading to the scission of the gly- cosidic bonds [2]. To track the depolymerization of cellulose in equipment, it is then important to identify the species that form following the scission of glycosidic bonds. In addition, these volatile species should possess partitioning characteris- tics that will favor their migration into the oil phase in order to be easily accessible for subsequent analyses. Information on the accumulation of such chemical indicators in the oil is es- sential for elaborating a predictive strategy that would guide the replacement planning of operating electrical units beyond their nominal design life. Until now, two main accelerated-ageing methods have been employed to examine the influence of temperature, oxy- gen, and moisture on the degradation of insulating papers and the resulting end-products. The first and most widely used method consists of carrying out heat-rise isothermal ageing tests on samples in air [3–7]. The main interest of this method is that it provides strict control over the environmental condi- tions such as the temperature and the initial moisture and oxy- gen contents of the samples. However, when the low tempera- tures are tested, i.e. 50–70 ◦ C, it can take several months and even years to significantly decrease the degree of polymer-