Factors Affecting the Reactivity of Zinc Oxide with Different Drying Oils: A Vibrational Spectroscopy Study

Abstract

This study focuses on the short-term reactivity of zinc oxide with different types of drying oils. Chemical grade zinc oxide (with diameter <100 nm) was mixed with oils with different proportions and types of fatty acids. Specifically, reactivity of zinc oxide with alkali-refined linseed oil, raw linseed oil, boiled linseed oil, polymerized linseed oil, stand oil, tung oil, and alkyd was studied between 4 days and 10 months. Effects of short-term exposure to temperature, humidity, and pH were also evaluated. The type of metal soaps formed and the kinetics of formation were monitored with attenuated total reflection Fourier transform infrared (ATR- FTIR) spectroscopy. The results confirm that humidity and heat combined favor the formation of both network-coordinated zinc carboxylates (POL-Zn) and zinc complexes of free fatty acids (FA-Zn) irrespective of the type of oil; however, the phenomenon is particularly severe for alkali-refined, raw, and boiled linseed oils, while stand oil and tung oil show the lowest amounts of zinc soap (FA-Zn) formation. The findings advance our knowledge of the reactivity of zinc oxide with an expanded range of drying oils that are commonly encountered in many nineteenth- and twentieth-century paints and reinforce the notion that even short-term exposure to heat with humidity can have irreversible effects on zinc carboxylate formation.