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Photo-aging Prevention of Paper-based Cultural Relics
試驗結果顯示出在過高的PEG含浸濃度（10 ％、15 ％、20 ％）時，高分子量PEG（10,000以上）的抑制光變色效果提升有限，且經過長期日照後，可能反而有發生紙張脆化之問題。因此，建議以低濃度之PEG含浸液（2 ％、4 ％、6 ％）、低分子量PEG（4,000、400）較適用於保存紙質文物方面。
In order to reduce the photo-aging of paper-based cultural relics, this study made some trials through physical prevention (blocking of oxygen and UV with polymer film or bag), and chemical processing (PEG impregnation). Furthermore, lignin-involved carbohydrates degradation has also been discussed.
1. Physical prevention
In this experiment, two methods were applied by using hot-sealing film and enclosure in ESCAL plastic bags to protect paper-based cultural relics from ultraviolet exposure and oxygen. These physical isolation processing could keep the brightness and paper strength to some satisfactory degree, especially good for lignin-rich paper such as newsprint. Taking out of oxygen and blocking UV irradiation, the effect of ESCAL bag on preserving carbohydrates' degree of polymerization was quite well.
In papers made of chemical pulp (Paper H and Paper F), the brightness could be held or even slightly photo-bleached after sunlight exposure, however, the physical strength of these papers did substantially decline. As a subsequent conclusion, the yellowing of paper should not suitably be the only criterion in the evaluation of photo-aging of papers.
2. Impregnation with PEGs
Exposing in the UV irradiation, the brightness of PEG-impregnated BCTMP hand sheet samples decreased remarkably in the first 5 hours, however, became gradually steady after 20 hours exposure.
The experimental results also showed that impregnation with high molecular weight PEGs (MW>10,000) in higher solution consistency (10 %, 15 %, 20 %) did not hold the brightness effectively. In addition, these PEG-impregnated samples became weak and brittle, mainly due to the long exposure of sunlight or UV, and behaved similarly to that of high molecular weight plastics.
Impregnation with low molecular weight PEGs (MW 4,000 and 400), and/or in low consistency of solution (2 %, 4 %, 6 %) are suitable for the preservation of BCTMP-containing papers from photo aging.
3. The degradation of lignin-involved carbohydrates
Attempting to clarify whether there is the possibility of lignin-involved carbohydrates degradation, impregnation of filter paper with milled wood lignin(MWL)and 3 lignin model compounds were processed.
After sunlight exposure, filter papers impregnated with MWL or lignin model compounds received more carbohydrates degradation in comparison with untreated ones. This result gave proof to show photochemical reaction of these lignin substances probably is the reason
of more intensive carbohydrates degradation and the weakening of paper strength.
In the FT-IR spectra analysis of sunlight irradiated MWL, the changes of organic functional groups match the photo-yellowing reaction mechanism of lignin as suggested by the former researchers.
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