The depth profiling of TiO2 pigmented coil coatings using step scan phase modulation photoacoustic FTIR

The depth distribution of a TiO2 pigment within the polyurethane (PU) coil coatings is investigated using step scan phase modulation photoacoustic (SS-PM-PA) FTIR. Coil coatings with different pigment contents were prepared and the modulation frequency (MF) of the SS-PM-PA FTIR varied to record the depth distribution of the pigment within the coating. The TiO2 pigment was shown to contribute significantly to the SS-PM-PA FTIR signal. A TiO2 aggregated region within the topcoat is found close to the topcoat-primer interface and further away from the topcoat surface. A deeper TiO2 aggregated region can be identified when pigment content is relatively low. The SS-PM-PA FTIR signal shows a considerable contribution from the primer originated signal, provided the TiO2 pigment content is sufficiently high and the modulation frequency applied is relatively low. SEM cross-section imaging results show a strong correlation of the TiO2 depth distribution with SS-PM-PA FTIR results, which confirms the applicability of the SS-PM-PA FTIR technique to the depth profiling study of TiO2 pigmented coil coatings.

► Step scan phase modulation photoacoustic (SS-PM-PA) FTIR reveals TiO2 depth profile.
► TiO2 aggregates close to topcoat-primer interface in coil coating topcoat.
► Lower pigment content gives deeper TiO2 aggregated region.
► High TiO2 content, low modulation frequency result in high primer signal contribution.
► TiO2 depth profiling is verified by scanning electron microscopy (SEM).