Study of the degradation mechanism of Chinese historic silk (Bombyx mori) for the purpose of conservation

Chinese historic silk fabrics dated from the Warring States Period (B.C. 475-B.C. 221), the Han Dynasty (B.C. 202-A.D. 220) and the Ming Dynasty (A.D. 1368-A.D. 1644) as well as artificial aged silk specimens were examined by SEM, TEM, amino acid analysis, FTIR, WAXD, SAED and DSC to investigate the degradation mechanism for the purpose of conservation. The variations of the relative concentrations of Gly and Ala and the crystallinity revealed that the degradation mainly occurred at the amorphous region at the initial degradation stage; both the amorphous and the crystalline regions were degraded at the same speed at the late degradation stage. Combined with the fact that the mechanical strength decreased over the degradation process, it could be concluded that the degradation of both the amorphous and the crystalline regions contributed to the decrease of the mechanical strength. During the degradation process, the morphology of the cross-section of the silk fiber was found to change from stacking particles to lamellar sheets, with decreasing size of segments of silk fibril. The changes of morphology and degree of orientation during the degradation process exhibited correlations with the change of secondary structure, which was greatly affected by aging method. The change of thermal stability exhibited correlations with both the secondary structure and the degree of orientation.