Kinetic Approach to Degradation of Paper In Situ FTIR Transmission Studies on Hydrolysis and Oxidation
| Main Authors: | , , , , |
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| Format: | Article |
| Language: | English Slovak |
| ISSN: | ISSN 1581-9280 |
| Online Access: | http://www.viks.sk/chk/preservscience2.doc |
| Abstract: | CONCLUSION.An in situ FTIR technique has been proven to give reliable data for the study of kinetics of paper degradation through hydrolytic and oxidative paths. The spectral analysis involved both qualitative interpretation of the observed intensities of carbonyl vibrations and carbonyl band assignment. To obtain kinetic curves of the degradation, our effort was focussed on standardization of the spectra, which included: - selection of a model paper for kinetic studies, - selection of the spectral region for kinetic interpretation between 1500-1900 cm where carbonyl groups occur, - elaboration of the experimental kinetic protocol with a particular stress on the determination of an initial state of the samples, - removal of water bending modes by collecting the spectra at an elevated temperature, - normalisation of spectra using an internal standard (CH 2900 cm" band of an initial sample), - relation of the Lambert-Beer's law to the conversion of carbon atoms in cellulose. In order to discriminate between oxidation and hydrolysis, the contrasting reaction conditions were chosen: dried air or water vapour, various temperatures, various amounts of Al added to model paper samples (P1). The reactions were regarded as predominating under the selected" conditions. The overall kinetic curves obtained show the differences in the degradation progress with time under various conditions. Their most characteristic features are as follows: - All kinetic curves started at the same initial conversion value which is characteristic of a paper type and its initial oxidation and polymerisation stage; -At lower temperatures (100 °C) hardly any changes in the time frame up to 150 h were observed for model paper samples P1 irrespective of the ageing atmosphere used. Al-modified Pisamples give much higher conversion than pure P1 samples. In water vapour the kinetic curves can be described by first order kinetic equation typical of hydrolysis. - At higher temperatures (1 50 °C), in water vapour atmosphere, a complex reaction pattern was exhibited by all the samples. For the P1 samples, the sigmoidal shape of kinetic curves can be accounted for by the autocatalytic character of hydrolysis. For the Al-modified samples a short fast acceleration period is followed by a constant degradation rate. In contrast, monotonous increase in the conversion was observed in dry air. - The correlation between Al content in paper (paper acidity) and the reaction rate under various conditions was also demonstrated A strong influence of Al content on oxidation rate at 150 °C implies that it is accompanied by acid-catalysed hydrolysis which supplies new centres for cellulose oxidation The carbonyl band assignment was supported by complementary results which allowed distinguishing carboxyl and aldehydic vibrational modes as arising at around 1740 and 1710 cm"1, respectively These also showed the most profound changes with time upon ageing under various conditions. All our findings are supportive of the parallel-consecutive mixed oxidation and hydrolysis mechanism involving autocatalytic path for hydrolysis. SUMMARIES.This study deals with quantitative and qualitative interpretation of transmission FTIR spectra of aged model paper samples (pure cellulose and cellulose modified with aluminium sulphate) to prepare the basis for a kinetic model of cellulose degradation involving a mixed hydrolytic and oxidative mechanism. The ageing experiments were performed in situ under various conditions (pure water vapour, dried air, 100, 150 °C) to discriminate between hydrolytic and oxidative paths. The focus was on the spectra between 1500-1900 cm-1, where the products of paper ageing appear in the form of various carbonyl groups. A procedure of spectra standardization allowed combining band areas with conversion of carbon atoms in cellulose. From the time evolution of bands the overall kinetic curves were generated and preliminarily analysed. The positions of carbonyl bands were verified by independent experiments. Basing on experimental results, a mechanism assuming parallel-consecutive oxidation and autocatalytic hydrolysis was proposed. |
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| ISSN: | ISSN 1581-9280 |
