Mechanical Degradation of Asian Lacquers
This research seeks to uncover the mechanisms by which surface cracks develop and propagate in lacquer coatings as a result of exposure to light and humidity fluctuations in order to improve their preventive conservation measures.
Asian lacquer is a natural product, which has been used for protective and decorative coatings since the second century BCE. It has been applied to wood, ceramics, metals, leather, and tortoiseshell. It is also used in the preparation of priming layers and subsequent foundation layers.
The production process consists of applying successive layers of sap from a lacquer tree onto a substrate. The polymerization of the sap results in the formation of a durable and water-resistant coating with a glossy appearance providing decorative effect.
Lacquer is susceptible to photo-degradation. Exposure to light and humidity fluctuations causes changes in its surface appearance, with a significant reduction in gloss. It is a result of the formation of micro-cracks in the surface, which in extreme cases can extend to foundation layers. Our current research focuses on uncovering the mechanism in which surface cracks are initiated and propagated in the material under unstable environmental conditions.
Although chemical degradation of lacquer coatings through photo-oxidation is well-know, its effects on their mechanical properties and the extent of changes occurring in the thickness-wise direction have been scarcely studied or quantified. The exposure to light causes low molecular weight compounds to vacate the material, resulting in shrinkage. Our preliminary nanoindentation studies show that this coincides with substantial stiffening of the material.
These facts allow us to hypothesize that light-induced shrinkage and stiffening of the surface layer, being restrained by the unaffected deeper layers of lacquer, result in building up of stresses, hence an increase in the accumulated elastic energy in the multilayered coating structure.
When the amount of accumulated elastic energy exceeds a critical value, the existing defects begin to propagate even without externally imposed deformation. However, the elastic energy being accumulated in the system due to light exposure alone may not be sufficient to initiate cracking of the material.
Further dimensional changes of the subsurface layer due to humidity fluctuations would thus be necessary to initiate the formation of surface cracks. This would explain why humidity variations often accelerate the degradation of light exposed lacquer coatings.
As a first step to verify the hypothesis, the mechanical properties of lacco/lacquer coatings, light aged to various degrees, are being studied at different relative humidity levels. The obtained nanoindentation results of the lacquer coating specimens, combined with their respective adsorption and desorption isotherms, will facilitate modeling of residual stresses that may develop in the material due to exposure to light and humidity fluctuations.
A separate attempt will be made to see if nanoindentation could be used to directly probe the residual stresses in lacquer coatings. Light exposure of lacquer can lead to the formation of microdefects on/in the surface, which can potentially serve as crack nucleation sites. Atomic force microscopy is being used to study the evolution of such local defects in lacquer as a function of light exposure.