Catastrophic Room Temperature Degradation of Cotton Cellulose
| Main Authors: | , , , |
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| Format: | Article |
| Language: | English Slovak |
| ISSN: | ISSN 0034-5806 |
| Online Access: | http//:www.viks.sk/chk/res_1_03_36_45.doc |
| Abstract: | SUMMARIES. Slow cellulose degradation at room temperature requires the application of accelerated ageing tests. After an ageing treatment, the paper samples were usually stored at standard room temperature, before being used for further studies. It was assumed previously that degradation was again very slow during the post-ageing storage. However, repeated DP measurements (6 and 18-20 months after the accelerated ageing) revealed a significant decrease in the polimerization degree, as measured viscometrically, for one of the tested papers (PAPER 2). Putting the aged paper in polyethylene bags prevented, or at least slowed down, the degradation process. During the accelerated ageing and storage-ageing, the pH of the paper decreased, as shown by the contact electrode measurements. Some conclusions, concerning the storage-degradation mechanism, have been drawn. This degradation was not a result of acid hydrolysis. It is very likely determined by an oxidation process leading to the formation of carboxylic groups. Chemical centres involved in the storage-degradation are formed during the accelerated ageing. Some of them are also destroyed later on, during the ageing. The nature of these centres remains unknown. After accelerated ageing, cotton cellulose degradation at room temperature is so fast (0.5 DP units per day) that the effect cannot be ignored. Any characterization of paper samples should be done as soon as possible after the thermal treatment. If this is impossible, the time lapse between the end of accelerated ageing and the date of a measurement should be recorded. DISCUSSION AND CONCLUSIONS. The presented data indicated that the rate of room-temperature degradation of cotton cellulose (PAPER 2), which had already been aged, was surprisingly high. In the most extreme case, after the shortest ageing time equal to two days, the degradation rate attained during the first half year of storage amounted to roughly a half DP unit per day. In the case of the softwood cellulose paper (PAPER 1) the corresponding value of DP change per day was importantly smaller and amounted to 0.05 units. This means that PAPER 1 was significantly more stable during post-ageing storage. Differences between the degradation rates of PAPER 1 and PAPER 2 have already been revealed in our previous article6. During the degradation of PAPER 2 the DP value decreased linearly with time (t), whereas for PAPER 1 the commonly used equation is valid, where DPo is the initial value of polymerization degree, and k is the rate constant. The information outlined above is a good starting point for the discussion on post-ageing room-temperature storage-degradation. During this process the DP neither decreased linearly with time (as shown by plots in Fig. 3) nor did it decrease according to eq. (1), as found by our calculations. Therefore it can be concluded that the storage-degradation was not a result of acidic hydrolysis. On the other hand, the data in Table 1 indicated that, putting aged PAPER 2 into polyethylene bags prevented, or at least slowed down, storage-degradation. Similar data for PAPER 1 were inconclusive. We speculated that this effect was due to the lack of oxygen and that storage-degradation was determined by an oxidation process. Oxygen may elucidate formation of carboxylic groups from hydroxyl groups. This should result in the enhanced acidity of the sample. This hypothesis can be supported by the increase of acidity with the ageing time and with storage time, clearly seen in Fig. 4. In order to get more information concerning the mechanism of storage-degradation from the experimental results, let us again take into account Fig 3. Let us define the following ratio: and the calculated values of r are as follows: We observed that: ˙ The rate of storage-degradation decreased significantly with storage time, as seen directly in Fig. 3. The curves approach some limiting value after 20 months of storage. This fact supports the applied approximate formula (3) for calculating r. ˙ The calculated value of r never exceeded 0.2, thus suggesting the existence of a limited number of potential centres active during the storage-degradation process. ˙ The value of r decreased with ageing time. This fact suggested that the real centres active during the storage-degradation process were formed during the accelerated ageing, although some of them were also being destroyed during this accelerated ageing. However, one should be aware that the nature of the above mentioned chemical centres remains unknown. In this context, essential differences between storage-degradation effects for PAPER 1 and PAPER 2 are worth emphasizing. Let us draw practical conclusions. After accelerated ageing, cellulose degradation at room temperature was so fast that the effect cannot be ignored. This implies that any characterization of the samples in question - e.g. spectroscopic or viscommetric - should be done as soon as possible after the thermal treatment. If, for some reason, this is not possible, the time between the end of ageing and the date of the measurement should be recorded. Needless to say that for the series of samples a uniformity of time gap is recommended. |
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| ISSN: | ISSN 0034-5806 |
