| Abstract: | CONCLUSION. There is ample evidence suggesting the importance of oxidative reactions during degradation of neutral to mildly alkaline paper. Using a variety of new analytical methods, a number of reactive intermediates were identified and even quantified, e.g. hydroperoxides and hydroxyl radicals. The initiating role of aldehydes was demonstrated and it was shown that a higher content of carbonyls leads to faster degradation of paper. The knowledge or reaction pathways is important in order to devise new stabilisation strategies. Removal of products of acid degradation (aldehydes and carbonyls among them) during deacidification is thus extremely important, in order not to increase the rate of oxidation after the paper is mildly alkaline. To do so, the deacidification medium should be able to dissolve and wash out the low-molecular weight hydrophilic compounds. The important role of water was outlined: - It is a reaction medium; - It is a plasticizer; - It facilitates dissociation; - It is a reactant. Its effect on oxidative degradation is thus complex and it was shown that the degradation rate at 80 °C is lowest at 20% and 95% RH. Adjustment of the storage conditions to optimal RH might easily lead to a substantial stabilisation effect. CaCO.rbased and MgCO3-based deacidification will lead to comparable stability of cellulose and bleached pulp models in conditions of storage in the usual humidity range, with MgCOj giving slightly better results. The alkaline sensitivity of some bleached chemical pulps is due to their high carbonyl content - such models should not be used for general studies of paper oxidative degradation. Further stabilisation may be achieved with stabilizers - their effect will be discussed in detail in Chapter 11. |
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