Characterization and evaluation of UV-irradiation aging of urushi by micro-UV/Py-GC/MS system

Lacquer is the main raw material of lacquerware, while UV-irradiation is an important factor leading to its aging. In order to investigate the aging behavior of urushi lacquer film under UV-irradiation, and to propose parameters that can be used to evaluate its aging degree, the micro-UV/Py-GC/MS system has been applied in the present work to enrich and identify volatile products released by urushi film during aging on the one hand, and to investigate micro-amount and conventional urushi films with different aging degrees on the other hand, in addition, representative historical samples have also been investigated. Volatile products released by urushi film demonstrate that during UV-irradiation, polysaccharides wrapping urushiol polymer would be first degraded to generate furans and alcohols, and then urushiol polymer would undergo photo-oxidation and break of molecular chain, mainly generating oxidation products including aldehydes, organic acids, and ketones, as well as a small number of alkyl benzenes and hydrocarbons. Py-GC/MS analysis of micro-amount urushi films with different aging degrees indicates that UV-irradiation is more destructive to aliphatic chains in urushiol polymer, but it also can accelerate the polymerization of micro-amount urushi film. Py-GC/MS analysis of conventional urushi films indicates that with the increase of aging degree, the total chromatographic peak area ratio of hydrocarbons to benzene derivatives gradually increases, the peak area ratio of 3-pentadecenyl-catechol to 3-pentadecyl-catechol gradually decreases, and the peak area ratio of aging products of urushiol to 3-pentadecyl-catechol gradually increases. The above parameters were further used to evaluate the aging degree of two urushi-contained samples separately collected from the surface and interior of a carved lacquer panel in the Forbidden City, and evaluation results are in accord with the natural aging law and further verify the reliability of the proposed parameters. Research results of the present work can lay a foundation for the in-depth study of the UV-irradiation aging mechanism of urushi.

Automated summary

This study used a micro-UV/Py-GC/MS system to investigate the volatile products released by urushi lacquer film during aging under UV-irradiation and proposed parameters for evaluating its aging degree. The results showed that UV-irradiation caused degradation of polysaccharides and photo-oxidation of urushiol polymer in the lacquer, generating various oxidation products. The proposed parameters were further validated through the evaluation of urushi-contained samples collected from a carved lacquer panel in the Forbidden City.