Gamma Radiation Treatment of Paper in Different Enviromental Conditions Chemical, Physical and Microbiological Analysis
| Main Authors: | , , , , , , |
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| Format: | Article |
| Language: | English Slovak |
| ISSN: | ISSN 0034-5806 |
| Online Access: | http://www.viks.sk/chk/res_2_01_107_131.doc |
| Abstract: | Summaries. A multidisciplinary team studied the effects of a decontaminating treatment with gamma radiation applied to a paper, which had been damaged by microscopic cellulosolytic fungi (mildews). Special attention was paid to the potentially negative effects of gamma rays on cellulose. The investigation was extended to the effects of some associated environmental treatments, such as the absence of oxygen and water saturation. The mechanical, chemical, physical and microbiological tests confirmed well-known observations already published. They indicated that the microbial population decreases proportionally to the gamma rays dosage together with the depolymerisa-tion of the cellulose molecule. Nevertheless, this negative effect does not significantly affect the basic properties of a good printing paper. A correct analysis of costs and benefits would suggest the use of a dose of 2-3 kGy. However, this analysis has still to be further investigated for the practical application of irradiation on deteriorated books and documents. Conclusions. We concluded from our experiments that an irradiation dose of 5 kGy caused statistically significant variations on almost all the paper properties. The treatment at 2 kGy, however, does not induce, in many cases, statistically significant modifications of the commercial paper. Generally, the vacuum (V) and the nitrogen (N) treatments did not reduce the negative effects of irradiation upon paper. As we expected, the S treatment modifies the paper properties. In fact, S samples have a moisture content higher than the Z samples when in equilibrium with the conditioning atmosphere (23° C 50% RH). These results were not surprising, as they were in accordance with the cellulose hygrometrical hysteresis16. Higher moisture content decreases the tensile strength while improving the stretch at break, the tearing resistance and the folding endurance. In the experiments with the accelerated aged samples it is impossible to reproduce exactly the effects of natural ageing. Nevertheless, the accelerated ageing method proved to be useful when investigating the effects of irradiation and the associated treatments on paper properties. Accelerated ageing usually causes a decrease of those mechanical properties that depend on fibre length and flexibility (tearing resistance, folding endurance). On the other hand, heat can increase the interfibre bonds and therefore the paper properties, depending on these bonds, can improve (tensile strength, bursting resistance). Unfortunately, the artificially aged paper gave quite variable results when tested: all considerations concerning the ageing effect are therefore subject to a certain error17,18. The colour of commercial paper was resistant to irradiation. Accelerated ageing caused the well-known yellowing effect in all samples. After ageing, the samples irradiated at 5 kGy showed a slight decrease of L* with respect to non-irradiated samples, suggesting that the effect of irradiation becomes visible only over time. Only after accelerated ageing and with an irradiation dose of 5 kGy the lightness (L*) slightly decreased. Nevertheless, this colour modification could not be detected by visual examination. The irradiated Whatman Paper showed a slight tendency to yellowing after accelerated ageing. The viscometric degree of polymerisation of Whatman Paper confirmed, as previously observed3, the depolymerising effect of gamma rays on cellulose. De-polymerisation occurs also after ageing, with a synergetic effect when associated to an irradiation treatment. The cellulose depolymerisation, however, caused by the low doses of irradiation which are sufficient for paper recovery, is still acceptable if we consider that this treatment would be used for paper materials already seriously infected. The advantage of irradiation with respect to other disinfectant treatments is that it does not leave behind any harmful residue, so that the subsequent handling of the documents is free of hazards. From the spectra obtained for Whatman Paper, some small oxidation differences are quantified spectrometrically in a maximum of 5% of CO mmol/g-1. The commercial paper, which is characterised by substantial signals of calcium carbonate, (content esteemed at 10-15%), shows after the treatment some small spectra differences, especially in the CHs and aldehydic groups, 2900 and 1360 cm-1, and in the alcoholic groups stretching zone, 1220 cm-1, that seem to point out to the prevalence of depolymerization compared to oxidation''. In fact, in the oxidised groups zone, 1735 cm-1, no absorption with a percentage of CO higher than those determined for Whatman paper are found, even in curve-fitting and Fourier deconvolutions. The small modifications in the commercial paper spectra are principally due to depolymerization and to the change of the relationship between its amorphous and its crystalline structure. These spectral changes can also be ascribed to the interactions between polysaccharides and other components. The very limited variations observed in Whatman Paper spectra are due to the poly-saccharide skeleton rearrangements, as also indicated by absorption signals at 1430 and 900 cm-1 20, 21. IR observations show that the studied papers do not show substantial structural changes and that the various associated treatments seem to have an irrelevant influence on the irradiation effects. The increase of the kappa number and of the light absorption coefficient, proportional to the absorbed dose, can be related to the DP decrease. Accordingly, the ISO brightness and the light scattering coefficient decrease when paper is irradiated, even if this is slight. The whole set of results on the fungi population survival clearly indicates that the gamma radiation effects are proportional to the absorbed dose. It is interesting to observe, that relatively low irradiation doses (2-3 kGy) decontaminate a paper, until the microbial presence reaches levels comparable, or even lower, to those found on the "blank" samples. This data seems to satisfy fully the objectives of a decontamination treatment. Such a treatment should not aim at a complete sterilisation of the contaminated material, which is not even necessary22, but at the removal of biodeteriogenous micro-organisms when they are too many, because of anomalous environmental situations. In the treatment of paper saturated with water (S-samples), a synergetic effect of the water during the irradiation phase can be observed. This may be explained by considering that when water is irradiated, free short-lasting radicals develop which have a germicidal power, added to the irradiation germicidal effect. For the other associated treatments (V = "Vacuum", N = "Nitrogen"), related to the absence of oxygen, the observed differences are negligible; they cannot justify an additional treatment. The differences in the microbial loads, also found at the same absorbed dose, can be caused by an accidental contamination, which may have occurred during the preparation of samples for the N and the V treatment, rather than to the effects of the treatments themselves. In fact, we have to keep in mind that for the V treatment it was necessary to create a vacuum, drawing out the air from bags containing contaminated paper samples. Similarly in the N treatment it was unavoidable to further disturb the environment, even if this should be avoided in microbiological experiments. Altogether it can be stated that irradiation does not damage the mechanical properties of paper, while it can perform a drastic reduction of the biological agents responsible for the material deterioration: not only microscopic fungi as those studied in this work, but above all insects which are more sensitive to radiation2. Nevertheless, before applying irradiation technology on a large scale for the recovery of damaged paper materials kept in libraries and archives, we intend in the future to make a more in depth study of the costs/benefits ratio for this technology with particular attention to the economical and engineering aspects. The advantage of irradiation with respect to other disinfectant treatments is that it does not leave behind any harmful residue, so that the subsequent handling of the documents is free of hazards. It is necessary to remember that when irradiation treatment will be used on books and paper documents to prevent their degradation, a carefully considered evaluation about the choice of the technologies of intervention22 will have to be done and it will be necessary to take care of other factors instead of only the material's answer to the treatment. Between these factors the following are decisive: the available alternatives, the entity and the typology of the biological attack, the urgency of intervention, the historical and commercial value of the material to be treated, the quantity, the economic and environmental costs of the treatment. |
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| ISSN: | ISSN 0034-5806 |
