We report on the accelerated ageing of cellulose based insulating paper by means of pulsed UV laser irradiation (λ = 248 nm)
under various experimental conditions including paper composition, background gas (He, N
2 and air) and moisture content of the paper. The temperature reached by the paper samples during their laser irradiation was
monitored by means of real-time IR imaging. It is shown that the equilibrium temperature (
T
eq) reached by the paper increases from ~30 to ~270 °C when the laser energy density was raised from 15 to 550 mJ cm
−2. The laser irradiated samples were systematically characterized by means of scanning electron microscopy (SEM) observations
and degree of polymerization (DP
v) measurements. Interestingly, it is found that, for a given moisture content, the degradation level of the cellulose is mainly
triggered by the
T
eq value reached during the laser irradiation. Moreover, their moisture content was found to influence significantly the number
of laser produced bond scissions (it doubles when the moisture content is increased from 0.5 to 6%); the paper degradation
is apparently not affected by the presence of oxygen as the background gas. These results suggest that the laser induced cellulose
degradation occurs through a direct photolysis (i.e. direct breakage of C–C, C–O and C–H bonds), leading to radicals formation,
which, in turn, are believed to induce the acid hydrolysis degradation mechanism, the latter being moisture dependent. The
activation energy (
E
a) of each gaseous species collected after the laser degradation was estimated. Their
E
a values were found to be in good agreement with the one associated to the laser depolymerisation of cellulose (i.e. ~56 kJ mol
−1), suggesting thereby a direct correlation between the cellulose degradation and the formation of the detected gaseous species.
Finally, the pulsed laser irradiation can be seen as an attractive tool to identify primarily generated molecules, on a very
short time scale, that can be used as relevant chemical markers for the monitoring of the ageing of transformers materials
with cellulose.
相似文献