Chemical and Mechanical Properties of Drying Oils during Polymerization

dePolo, Gwen  Ellen

doi:https://doi.org/10.21985/n2-svn5-nm47

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Chemical and Mechanical Properties of Drying Oils during Polymerization

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Drying oils have been used as a binding medium for oil paints since the 15th century. These oil paints transition from a liquid-like paste to a solid-like film as a result of crosslinks forming between the oil molecules. These reactions have been extensively studied chemically, but other material properties are not as well characterized for drying oils. Oil paints are typically used in complex composite structures where the mechanical properties matter just as much as the chemical properties. This thesis focuses on understanding the evolution of mechanical properties during drying oil polymerization. An overview of mechanical testing techniques used to study artists' paints in the last few decades is provided in Chapter 2. The effects of temperature, composition, relative humidity, and solvent exposure on the mechanical properties of a paint are discussed. Newer techniques that have the ability the monitor the evolution of mechanical properties in cultural heritage objects are also presented. One of the challenges for most mechanical testing techniques is that they are unable to follow the evolution of mechanical properties during drying oil polymerization. In Chapter 3, the quartz crystal microbalance (QCM) is able to fill this gap, allowing changes in mass and mechanical properties to be measured during polymerization. Linseed oil samples are measured to observe these changes over several years of aging. Litharge (used to speed up polymerization) is incorporated and its effects are quantified. The temperature dependent response of a linseed oil film are measured and the lack of a well-defined glass transition temperature for the film implies that there is nano-scale heterogeneity due to variations of crosslink density in the film. Another aspect that is not well understood is the effect of fatty acid distribution on drying oil polymerization. Chapter 4 discusses the use of time-based attentuated total reflection infrared spectroscopy to obtain higher resolution of the chemical changes occurring during initial drying oil polymerization. Five drying oils are measured and demonstrated that there are some chemical differences depending on the fatty acid distribution of the drying oil. Incorporating litharge results in a significant reduction in the levels of oxidation reached in the drying oil films.

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