(122d) Examining the Initial Curing Mechanism of Ethyl Linoleate Using Monomer-Based Kinetic Monte Carlo Simulation

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. Due to the complex chemistry and crosslinked polymer, there is an incomplete understanding of the autoxidative curing mechanism, so microkinetic modeling is used to offer insight into this dynamic chemical system on the molecular and macroscopic levels.

This work overcomes previous modeling limitations by leveraging a multifunctional monomer graph representation of polymers and kinetic Monte Carlo (kMC) simulation to track a vast set of chemical and polymeric species in time. A monomer-based approach maintains the atom-level detail of monomers and their reactivity. The polymer is then constructed as a network graph of these monomer units with crosslink detail maintained in the edges that connect the monomer nodes. The main curing pathways have been elucidated by experiments and recent kinetic modeling efforts; however, the complexity of the polymer system limits further investigation of the secondary reaction products.

A kinetic model of ethyl linoleate autoxidation is presented, which includes key reaction families such as radical initiation, bi-allylic hydrogen abstraction, Co-catalyzed hydroperoxide decomposition, radical addition, β-scission, recombination, disproportionation, and epoxide ring formation. In literature, epoxide ring opening is also found to be a significant secondary reaction pathway within the curing of ethyl linoleate; however, there is a lack of kinetic data to elucidate a rate constant for epoxide ring opening.[1] Thus, the mechanistic model is extended to include ring opening and examine proposed pathways. The model is validated against experiments from literature on ethyl linoleate curing.[2] The concentration of functional groups of interest over time, crosslink distribution, and molecular weight distribution are presented.

This work 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] Muizebelt, W. J.; Hubert, J. C.; Venderbosch, R. A. M. Mechanistic Study of Drying of Alkyd Resins Using Ethyl Linoleate as a Model Substance. Prog. Org. Coatings 1994, 24 (1), 263–279.

[2] Oyman, Z. O.; Ming, W.; van der Linde, R.; van Gorkum, R.; Bouwman, E. Effect of [Mn(Acac)3] and Its Combination with 2,2′-Bipyridine on the Autoxidation and Oligomerisation of Ethyl Linoleate. Polymer (Guildf). 2005, 46 (6), 1731–1738.