聚乙烯醇和聚醋酸乙烯在特殊环境中的光照老化试验

Results of an Ⅲumination Ageing Test using Polyvinyl Alcohol and Polyvinyl Acetate under Controlled Conditions

前言聚乙烯醇和聚醋酸乙烯是敦煌壁画的主要修复材料。针对其存文物修复中的主要应用特性,已作了许多研究工作。作者曾在前一阶段,就这两种材料成膜后在窑气中的老化情况,进行了人工加速老化试验。试验结果表明:聚乙烯醇和聚醋酸乙烯薄膜在相对湿度为0、30％、50％、75％、100％的空气中,常温下经历了相当于莫高窟洞窟中2年和9年光照能量的人工光照老化试验后,其化学结构没有明显的改变,显示了较好的耐光性和对各种温度的适应能力。

polyvinyl alcohol and polyvinyl acetate are the two chemicals which have been most widely used in the conservation of frescoes in Dunhuang to date. The results have been positive. The polyvinyl alcohol or the polyvinyl acetate forms a thin film on the surface of the fresco after it has undergone repair. The tests were conducted in the following sequence: One lot of specimens ,covered by the two different polyvinyl chemicals were exposed to oxygen or carbon dioxide under controlled humidity. To promote ageing, strong illumination was used. An analysis was then made and evaluated. A second lot of specimens with red and white lead added to the film (i. e. red and white lead are the two pigments which change colour most easily) were also exposed to the above mentioned controlled conditions and later analysed and evaluated. The ageing process was carried out in air-tight dissicators, which contained either oxygen at half of atmospheric pressure or 4,000 ppm carbon dioxide. The relative humidity rates used were 10%,30%,55%(60%)and 90%. The luminous intensity of the daylight lamps was 4,500 LUX. The ageing time period was 83 or 159 days respectively, which is equivalent to four to eight years inside a Mogao Grotto cave. Results of the exoeriments conducted, indicated that all the aged specimens studied under a humidity rate of 60% and above, showed only slight signs of shrinkage or wrinkling; other rates of humidity showed nooutward changes. The infra-red spectrum analysis of aged specimens showed no obvious changes in the genes or any signs of new genes being born. It showed stability and properties of anti-oxidity and anti-acidity. As regards the colour of specimens after a film of red lead was applied, it became slightly pale with ageing, when the humidity rate was increased, however, no change in the chromatic range was seen. Analysis by x-ray diffraction showed hardly any trace of pbo_2 in the aged specimens. The same can be said of the specimens using a film to which white lead had been added.