Yellowing of Modern Papers
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| Format: | Article |
| Language: | English Slovak |
| ISBN: | ISBN 0-8412-0553-1 |
| Online Access: | http://www.viks.sk/chk/ppthav_119_141.doc |
| Abstract: | Summaries. Paper in its applications must satisfy a number of criteria, and of these, permanence and durability are the most important ones. Unfortunately, modern papers produced from wood pulps contain a high quantity of vulnerable cellulose and hemicellulose that have been oxidized during the pulping processes. They are relatively unstable toward light, heat, moisture, air, and other environmental factors. In addition, because of the shortage and high cost of raw materials, modern "high-yield" papers contain oxidized cellulose and hemicellulose; they also incorporate a considerable amount of lignin and extraneous material from wood, which further impairs the paper quality-permanence and durability. In this chapter, discoloration or yellowing of modern papers as a consequence of the oxidative reactions of cellulose, hemicellulose, and lignin as well as color stabilization is reviewed. Suggestions for further research in color stabilization are made. Conclusion. From the preceding discussion, it is obvious that discoloration or yellowing of modern papers is a complex set of reactions depending upon the historical background of the materials, methods of fiber and paper production, and the conditions of their usage and storage. "Pure" cellulose and hemicellulose should be unaffected by exposure to natural sunlight. It generally is believed that it is the presence of impurities in the commercial cellulose, introduced during pulping processing or autooxidation, that results in light absorption by the polymer and that may initiate photochemical reaction. The major chromophoric groups are believed to be carbonyl and carboxyl groups, hydroperoxides, and metallic impurities. Lignin is sensitive to sunlight. The -carbonyl groups, conjugated double bonds, quinonoid structures, and phenolic hydroxyl groups in lignin are the vulnerable functional groups that absorb harmful incident radiation leading to photochemical reactions. The photoinduced discoloration of modern paper is enhanced further by the presence of paper additives and impurities as well as the presence of moisture, heat, and oxygen. To effectively prevent photoinduced discoloration of modern paper, one must consider the environment m which it may be stored, exhibited, or used, and the causes of its yellowing. In view of the photochemistry involved, some form of photostabilization of modern paper is essential if adequate protection against the detrimental effects of solar irradiation is to be achieved. Principally, the photostabilization of modern paper involves the retardation or elimination of the various photophysical and photochemical processes that take place during photoinduced discoloration. This goal may be accomplished by the following three stabilizing systems. 1. Physical Modification of Modern Paper. This system relies on the presence of stabilizers for its stabilizing action. The stabilizers are of four general types (177,178,179): UV screeners, UV absorbers, quenching compounds, and free radical scavengers. By strict definition, in this case, UV screens are materials that are interposed as a shield between the modern paper and the radiation source. They function by reflecting the damaging incident light, thereby limiting its penetration into the bulk of the paper. Ultraviolet absorbers function by absorbing and dissipating UV irradiation that would otherwise initiate discoloration of modern paper. Quenching compounds function by dissipating the excited energy from modern paper. Free radical scavengers act as a trapping means to catch the free radicals generated in the modern paper before they stabilize themselves as colored end products. 2. Chemical Modification of Modern Paper. Chemical modification encompasses both the purification of modern paper and the alteration of chromophoric configurations of the chemical constituents in paper to produce the desired pho-tostability. Impurities that enhance the photo-induced reaction can be foreign materials incorporated during processing (such as metallic impurities), or they can be an integral part of paper components (such as carbonyl or carboxyl groups). Consequently, elimination or reduction in the amount of impurities, either foreign or as part of paper components, is a practical approach to stabilization. Furthermore, elimination of chromophoric systems in the paper components by introducing new functional groups that are insensitive or less sensitive to light is essential. Benzolation, methylation, acetylation, and sodium borohydride reduction demonstrate such improvement, yet most of these processes are too expensive to be practical. Development of new processes that are economically feasible is required. 3. Combination of Physical and Chemical Modification of Modern Paper. The photostabilization of modern paper can be further improved by combination of the above-mentioned two systems. The addition of stabilizers to chemically modified modern paper can avoid further photo-induced reactions through the incomplete elimination of the chromophoric moiety in the paper. |
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| ISBN: | ISBN 0-8412-0553-1 |
