Photochemical degradation of cellulose

Many common metallic impurities in paper, particularly compounds of some of the transition metals, contribute to degradation of cellulose by hydrolytic or oxidative reactions. The more important in commercial papers are iron and copper compounds, whereas some others such as magnesium compounds have been observed to exert protective effects (7). It is clearly desirable that the content of undesired metallic ions be kept low in permanent papers. Titanium dioxide, commonly used as a filler, has been observed to promote degradation by photochemical reactions. The predictive potential of metallic content in relation to permanence, however, does not allow the setting of permissible limits at the present time. 

Several years ago we began to express the results of our thermal and photochemical degradation experiments in terms of Equation 4. Figure 1 shows some of the initial data obtained, relating to exposure of test sheets in the Fade-ometer. Just as Daruwalla and Narsian (16) observed, there seems to be a break in the curve. If the first stage indeed represents weak links, about 3 such bonds per 10,000 in the original cellulosic chain (0.03 ) are apparent, a figure similar to the fraction of labile acid-sensitive links determined by Sharpies (21) and labile photochemically sensitive links reported by Daruwalla (16) when radiation at 2540 A was used. 

Although the concepts outlined in this chapter are particularly appropriate for the interpretation of hydrolytic deterioration of cellulose, they show promise as an aid in the interpretation of thermal, photochemical, photolytic, and enzymatic degradation as well. Equations 3 and 4 are generally applicable to the scissioning process in linear polymers (33, 34). 

The major causes of instability of cellulose nitrate are due to the products of hydrolytic, thermal and photochemical reactions. Degradation of the polymer is autocatalytic, that is, the products of breakdown tend to catalyse a faster and more extensive degradation reaction than the primary processes, if allowed to remain in contact with degraded cellulose nitrate. 

However, cellulose is a relatively stable material. In lignified cellulose, such as wood flour, the degradation of lignins dominates the photochemical processes. Most of the lignin is lost from the wood surface during UV exposure, and the depth of degradation is in the range of 0.5-2.5 mm (0.02"-0.1") [9]. 

Major components of wood are cellulose, lignin and hemicellulose, and all of them are susceptible of photochemical degradation, behavior manifested by discoloration and loss of gloss and lightness, roughening and loss in mechanical properties [22, 36]. 

When cellulose fibers were partially photochemically degraded by exposure to UVB radiation prior to their use in composite formulations with PP, they proved to enhance the biodegradation of composites as their surface became more hydrophilic after exposure [64]. 

Natural Polymers.—The photolysis of tyrosine and its derivatives has been studied in methyl cellulose films." The derivatives were classified as follows according to their photochemical reactions tyrosine and tyramine degraded without the generation of any new, longer-wavelength-absorbing products the... 

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