Gamma irradiation of Washi -: part 2:: Changes in degree of polymerization and crystallinity of cellulose

To simulate the properties of old, i.e. naturally aged paper, so that it would be suitable for the restoration of historical paper objects, modern Japanese paper (Kozo) was subjected to accelerated ageing techniques: thermal ageing at 80degreesC and 60% RH over periods of 1, 2, 3 and 4 weeks, and irradiation with a cobalt 60 source with doses of 49, 98, 195 and 390 kGy over a total period of 48 hours. During thermal ageing degree of polymerization changed gradually with time, while it changed drastically with increased irradiation dosage. The strength of the samples changed noticeably only when the DP dropped below ca. 700. The crystallinity of kozo fiber was increased by gamma irradiation and thermal ageing, and paralleled the decrease of DP. These changes were most noticeable when irradiation followed thermal ageing. They explain well the decrease of physical strength reported in our previous paper.