The Chemical Drying Process in Alkyd Emulsion Paint Films

Abstract

This paper deals with the oxidative drying of films (dry thickness 100 mm) made of an alkyd emulsion based on a high viscosity resin. Half of the soybean fatty acids in this resin were partly conjugated. The oxidation process is studied by monitoring the disappearance of the double bonds both the conjugated and the non-conjugated cis bonds. This was achieved by using by spatially resolved Confocal Raman Microscopy. The principle and some technical limitations of Confocal Raman microscopy are briefly discussed. The effect of various driers is reported. The results show a number of interesting features. Apart from the expected rapid disappearance of double bonds near the surface, the double bonds also disappear fairly rapidly and uniformly in a slab of 60 mm at the substrate side of the film, depending on conditions. It is only slightly visible with manganese drier, pronounced with cobalt drier and very pronounced with a cobalt zirconium combination drier. Additionally, with only cobalt the conversion is also uniform in a surface slab of a thickness of 10-15 mm. The experimental data were complemented by computer simulations of the combined reaction / oxygen diffusion process. A model is presented to explain the disappearance of double bonds in the absence of new supply of oxygen. Unless a cis-cis set of double bonds, a conjugated set of double bonds can become saturated without the consumption of radical activity. On the basis of the used theoretical model, time and length scales of the diffusion reaction mechanism are discussed