Light Induced Degradation of Nitrocellulose Lacquer Thin Films for Conservation of Art Objects
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MRS Advances © 2017 Materials Research Society DOI: 10.1557/adv.2017.597
Light Induced Degradation of Nitrocellulose Lacquer Thin Films for Conservation of Art Objects Rebekah I. Webster and H. Francis Webster Chemistry Department, Radford University, Radford, VA 24142, U.S.A
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.
Introduction Nitrocellulose (NC) lacquers have been used by conservators as protective coatings for a variety of surfaces, including silver, bronze, and wood, among others. Nitrocellulose coatings have been reported to better protect silver when compared to other synthetic polymers and coatings can be applied by brushing, dipping, or spraying [1]. It is well known, however, that nitrocellulose will photochemically degrade in a series of complex mechanisms that depend on the intensity and wavelength of light [2,3]. While detailed studies have been conducted to fully understand the thermal decomposition of nitrocellulose due to its importance as an explosive propellant, there are relatively few studies focused on the decomposition of nitrocellulose by light. Fewer still concentrate on thin nitrocellulose-based coatings on metal substrates. The photooxidation of NC has been described as an autocatalytic process involving rate limiting primary decomposition steps with NO2 loss followed by secondary decomposition steps where NO2 and other reactive intermediates react with NC to accelerate the polymer degradation [3,4]. These reactions have also been shown to
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result in the formation of highly oxidized products [4,5] and polymer crosslinking [6]. While such degradation pathways are very complex, the kinetic results are often fit to simple zero or firs
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