Depth profiling of clear coil coating by confocal Raman microscopy

Confocal Raman microscopy (CRM) has been demonstrated to be very effective in the analysis of the distribution of chemical moieties within polymeric coil coatings. To verify the compatibility of CRM with commercial coil coatings, a multi-layer coil coating system was prepared and depth profiled by CRM with both dry and immersion oil objectives (non-destructive method). The cross-section of the same sample was then scanned by CRM (destructive method). It was found that the CRM depth profiling resolution was affected by both the intrinsic and extrinsic factors of the objective lens. The use of an immersion oil objective improved depth resolution and minimised the refraction effect, however the oil contaminated the coating surface. The dry method yielded the lowest depth resolution but was completely non-destructive. The CRM lateral scanning of the sample cross-section yielded the most accurate depth profile information; however, the destructive nature of this method is a major disadvantage. It was also found that pigments incorporated in the coil coating formulation affected the CRM depth profiling accuracy due to the strong Raman scattering of these materials.