Probing organic‐inorganic interactions and curing processes in coatings by photoacoustic fourier transform infrared spectroscopy

Photoacoustic Fourier transform infrared spectroscopy (PA FT‐IR) was utilized to follow the curing processes in pigmented linseed oil based coatings deposited on a substrate. This nondestructive technique allows us to monitor both chemical reactions and processes such as solvent evaporation and pigment stratification during auto‐oxidative curing. Spectroscopically detected solvent evaporation is correlated with the weight loss measurements. Apparently, only a small portion of the solvent molecules evaporates during the curing process and the remaining fraction participates in the crosslinking process. The interactions between organic (binder) and inorganic (pigment) phases of the coatings are also detected. It is found that in order to obtain a uniform distribution of the inorganic phase across the coating, it is necessary to consider both the hydrogen bonding between the two phases and the gravitational forces imposed on the pigment particles. Essentially, a lack of hydrogen bonding leads to stratification of the pigment phase and pigment volume gradient (PVC) across the coating. These processes are monitored spectroscopically and indicate that the hydrogen bonding is equally significant with other processes such as crosslinking and degradation.