(597e) Kinetic Monte Carlo Simulation to Study the Initial Curing Phase of Model Paint Films

Oil paintings are complex, reactive systems, particularly during the transformation from wet paint to a dry work of art. Linseed oil, an unsaturated triglyceride and common paint binder, cures by autoxidation which links the monomeric fatty acids into a polymer matrix that increases in functionality over time. What happens in this initial curing phase impacts the composition, reactivity, and stability of a paint film. Physical experiments fail to provide enough detail to understand the many competing reactions, so microkinetic modeling is used to offer insight into this dynamic chemical system.

This research implements kinetic Monte Carlo simulation with a monomer-based representation of polymer species to track their evolution over time. Chemical species are described as a graph of monomer nodes with atom-level resolution connected by covalent bonds. As a result, species have both coarse and fine levels of chemical detail, and methods from graph theory can be applied for facile interpretation and comparison of polymers.

In a toy model, ethyl linoleate, a doubly unsaturated fatty acid ester, undergoes autoxidation catalyzed by cobalt 2-ethylhexanoate, a common drier. Major reaction families included in this model are hydrogen abstraction, addition, radical recombination, Co-catalyzed oxidation, β-scission, disproportionation, and cycle formation. Preliminary results suggest that the monomer-based stochastic approach is effective at capturing both key and peripheral species without a combinatorial explosion as seen with rule-based automated reaction network generation in which all molecules and reactive intermediates are monitored explicitly. The concentrations of functional groups and measures of interest such as peroxide value and oxygen uptake are compared to experimental results by Oyman et al.[1] Future work will extend the chemistry and investigate the effects of temperature and drier concentration on functional groups of interest.

By studying a model of how oil paint dries on the molecular level, this research can investigate the complex reaction mechanism and identify deficiencies in our chemical understanding. A well-designed model system is motivated by applications in cultural heritage research and developing a non-invasive tool for art conservators to use a chemically equivalent simulated paint sample to test cleaning techniques instead of relying on artificial aging at elevated temperatures to accelerate natural phenomena.

[1] Oyman, Z. O., Ming, W., van der Linde, R., van Gorkum, R., & Bouwman, E. (2005). Effect of [Mn(acac)3] and its combination with 2,2′-bipyridine on the autoxidation and oligomerisation of ethyl linoleate. Polymer, 46(6), 1731–1738.