| Abstract: | CONCLUSION. Sugars are among the potential products resulting from the deterioration of cellulose and of the hemicellulosic constituents of paper. Four commonly encountered aldoses are extensively discolored by heat: xylose and arabinose discolor more extensively than glucose or galactose. Thermal aging under humid conditions results in far greater discoloration than under dry conditions. In contrast to thermally induced discoloration, papers impregnated with sugars are scarcely discolored by exposure to the radiation from "daylight" fluorescent lamps or from near-ultraviolet-emitting BLB fluorescent blacklights. The slight darkening that occurs under these circumstances can possibly be attributed to a certain degree of thermally induced darkening. Thermal discoloration of these materials under moderate conditions tends to be greater in acid conditions. Alkalinity much above pH 9 tends to cause immediate darkening which is, however, readily bleached upon exposure to "daylight" fluorescent lamplight. The principal conclusion to these investigations is that the thermally induced discoloration of aldoses and related sugar acids is at least partially bleached by exposure to "daylight" fluorescent lamps. If such materials should be responsible for some of the discoloration of aged paper products, the evidence is that they would be extensively, although perhaps not completely, bleached by exposure to either visible or near-ultraviolet radiation. In contrast, compounds similar in structure to substances that might be found in lignin are not so much discolored by heat as by exposure to ultraviolet radiation. In neither case did the bleaching by visible radiation of thermally discolored products from sugar nor of photochemically discolored model compounds related to lignin appear to go to completion even after rather extensive exposures. This is an important practical aspect of light bleaching that must be verified by further experiments. Nonetheless, this investigation has demonstrated that the bleaching effect of exposure to "daylight" fluorescent lamps, with 96.2% of the radiant power in the visible wavelengths, is most effective against the thermal decomposition products from sugar or sugar acids and is least effective against photochemically discolored substances derived from model compounds related to the components of lignin. |
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