The influence of the pigment volume concentration on the curing of alkyd coatings : a 1D MRI depth profiling study

The level of pigmentation varies in different types of alkyd coatings. Obviously, the properties of a coating are influenced by the pigment volume concentration (PVC). Especially, around the so-called critical pigment volume concentration (CPVC) the properties change dramatically, since the coating structure changes considerably around this point. Although it is known that the permeability changes at the CPVC, the effect on the chemical drying of alkyd coatings as a function of depth has never been investigated because of the lack of experimental techniques. For the first time 1D high spatial resolution magnetic resonance imaging (MRI) is applied to monitor the curing of alkyd coatings with different degrees of pigmentations as a function of depth. The MRI depth profiles clearly show a change in curing behavior around the CPVC. Below the CPVC a macroscopic reaction front is observed, of which the speed does not depend on the degree of pigmentation and moves with View the MathML source. Such fronts develop because the cross-linking process is limited by oxygen diffusion towards the reaction front. Above the CPVC a coating seems to cure homogeneously. This sudden change in curing behavior is attributed to the existence of coating spanning pore networks above the CPVC. Therefore, oxygen easily penetrates the complete coating. SEM images confirm the existence of porosity at high degrees of pigmentation. Since above the CPVC the reaction still proceeds with View the MathML source, it is assumed that on a local scale reaction fronts also develop and the curing speed is limited by oxygen transport. Due to the presence of the porous network the reaction front has complex 3D shape and fills the complete pore space. Therefore, the curing process seems to occur homogeneously above the CPVC.