Significance of surface and bulk light scattering in microgloss and microgloss nonuniformity of coated papers: Influence of pigment characteristics and calendering conditions

Abstract A custom-made imaging reflectometer was used to study the microgloss properties of coated papers. To quantify the relative contributions of surface and bulk scattering to the total intensity of reflected light, a pair of polarizer lenses was employed. The mean value and the standard deviation of local grey level of captured images at micro-scale (with a spatial resolution of 16 μm x 16 μm) were determined to quantify the average microgloss and microgloss nonuniformity of samples, respectively. It was found that surface microgloss; i.e. local light reflection in the specular direction due to surface light scattering, increased linearly with increasing the total microgloss of paper. However, bulk microgloss; i.e. local light reflection in the specular direction due to the bulk scattering, remained nearly constant for a wide range of coated papers with varying coating weight, formulation and calendering condition. The relative contribution of bulk scattering to the total microgloss increased as the sheet progressively became less glossy. On the other hand, microgloss nonuniformity was governed by the local variation in surface light scattering and was affected by pigment particle size, pigment morphology and calendering conditions. Finally, both surface microgloss and surface microgloss nonuniformity were power-law functions of the surface texture parameter (L_c/λ)/(σ_h/λ)²where L_c, σ_hand λ are correlation length, RMS surface roughness, and wavelength of incident light, respectively.