| Abstract: | SUMARRIES. The application of accelerated ageing experiments at an elevated temperature for modelling the ageing behaviour of paper, especially alkaline paper, has long been questioned (Porck 2000). Although the ISO Standard 5630/3 prescribes the ageing conditions for paper (80 °C at 65% RH), the result of such an ageing experiment for alkaline cellulose impregnated with iron chelator diethylentriaminepentaacetic acid (DTPA) was shown to be in a contradiction with the expected behaviour of the sample at room temperature, as it was shown in a recent study (Strlic et al. 2000 a). Indeed, in a multi-reaction system like cellulose autoxidation it is difficult to expect that a single accelerated ageing experiment can provide a good estimation of the ageing behaviour at the temperature of use. A set of experiments has to be performed at various temperatures in order to be able to obtain an apparent activation energy Ea for the rate constant of material degradation k from the corresponding Arrhenius plot: ln(k) = -Ea/R ˙ T. Once equipped with the apparent activation energy, it is possible to calculate the value of the measured material property at the temperature of use from the above equation, i.e. to predict the ageing behaviour of the particular paper. However, a typical ageing experiment at 50 °C may last up to a few months, depending on the degree of stabilisation of a particular sample, and experiments at still lower temperatures are desirable in order to obtain better quality data. Since such studies may become extremely time-consuming, other solutions are intensively searched for. It has long been known that emission of light, reffered to as chemiluminiscence (CL), accompanies some chemical reactions and studies of chemiluminescence accompanying degradation of polymers have been attracting the attention of chemists for almost four decades (Ashby 1961). However, for various reasons, the most obvious one being the complexity of data obtained, the technique has never been widely used. Even so, it contributed a vast amount of knowledge on the degradation mechanisms of artificial polymers (Zlatkevich 1989). Although not routinely in use, the lifetime predictions obtained by chemiluminescence studies have been shown to correlate with accelerated ageing studies for some artificial polymers (Billingham 1999). Since such experiments are not nearly as time-consuming as accelerated ageing and since due to the extreme sensibility of the present-day CL instruments the reaction temperature may be significantly lower than the ISO Standard 5630/3 temperature, we are faced with a possibility to complement the existing accelerated ageing studies and obtain more reliable results more quickly. The temperature, at which the measurements (or accelerated ageing) are performed, is vital: the lower the temperature (or: the closer to the temperature of use) the more likely is the expected performance of material to correlate with performance at the conditions of use. The elegance of the chemiluminescence technique is that we can follow the chemical changes in situ, i.e. while the very reactions take place, with the help of a photomultiplyer. Another advantage is its ability to provide valuable information on the initial stages of polymer degradation. For the reasons discussed, chemiluminescence measurements may provide some advantages over the conventional paper ageing techniques. In the area of conservation research, more refined analytical methods for comparative investigation of paper ageing are needed (Porck 2000; Pedersoli 1999), and we shall briefly review the general principles, the present state of knowledge, the future potentials of this technique and discuss the applicability of the method for a paper chemist and conservator. |
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