Investigation of three-dimensional spatial structure of iron oxide alkyd paint by X-ray nano-phase tomography

The spatial dispersion of iron oxide pigments plays a critical role in determining the uniformity and functional performance of alkyd resin-based coatings. However, conventional two-dimensional (2D) characterization methods are often insufficient for resolving the three-dimensional (3D) aggregation state, particle morphology, and spatial arrangement of pigments within cured coating films. In this study, ptychographic X-ray computed tomography (PXCT) was employed to investigate the 3D microstructure of iron oxide alkyd coatings prepared under different mixing conditions. Quantitative analyses were conducted to characterize particle & cluster fractions, particle & cluster morphology, and inter-particle distance distribution. Compared with the 3 min mixing, the 1 h mixing treatment increased the volumetric fraction of dispersed particles from 51.09% to 76.04%, and reduced the cluster fraction from 48.91% to 23.96%. In addition, the mean inter-particle distance decreased from 14.02 to 12.69 μm. These results indicate that stronger and longer mixing promotes cluster disaggregation and leads to a more homogeneous distribution of pigment particles. This work provides a quantitative 3D approach for revealing pigment dispersion in alkyd coatings, and helps clarify its influence on coating structure uniformity and performance.