Light Induced Degradation of Nitrocellulose Lacquer Thin Films for Conservation of Art Objects

Abstract

To better understand light induced changes in polymeric coatings commonly used to protect metal artifacts, this work details the decomposition of thin (~100nm) nitrocellulose-based lacquer films on silver, copper, and gold. Samples were exposed to various wavelengths of light including, UVA (~340nm), UVB (312nm), and UVC (254nm), under controlled temperature and humidity. Decomposition was characterized using reflection absorption spectroscopy (RAS) and monitoring infrared absorbance bands. The loss of NO₂ functional groups was observed and the decrease was fit to a simple two step autocatalytic model involving primary and secondary decomposition steps. The decomposition rate under UVC light exposure was rapid and similar for all substrates. Rate constants for the primary and secondary steps were similar in magnitude and only mildly dependent on temperature. For UVB light exposure, all reaction rates were much slower and secondary decomposition was much faster relative to the primary decomposition step. Decomposition under all conditions resulted in the formation of oxidized species with an infrared spectrum similar to that of carboxylic acids.