Texture and orientation of kaolin in coatings

X-ray diffraction (XRD), infra red (IR) spectroscopy, gloss goniophotometry and stylus profilometry were used to investigate the alignment of kaolinite applied as a thin coating on a polyester film substrate. The principles and applicability of the analytical methods are reviewed and discussed in the context of a kaolinite coating. X-ray diffraction and transmission IR were used to measure kaolinite misalignment and orientation distribution in the coating. Transmission IR measures the coating bulk; XRD data are surface biased, but contain contributions from the bulk. Attenuated total reflectance-IR (ATR-IR) provides a direct measure of kaolinite alignment within ~1 µm of the coating surface and also allows an assessment of surface smoothness. Gloss goniophotometry and contact profilometry measure surface microroughness and macroroughness, respectively, rather than kaolinite particle orientation. However, the properties of roughness and orientation are related. Bulk and surface texture are not necessarily correlated and a combined approach using both bulk and surface sensitive techniques is required for a full understanding of coating structure.