Surface Reflection Increase
An increase in the percentage of light reflected at the specular angle from a paperboard surface occurs when the surface micro-roughness is reduced. This specular gloss elevation is typically achieved through intense calendering or by applying smooth, high-solid clay coatings. It is a primary target for high-quality packaging where a shiny, reflective finish is desired for print contrast.
Surface Compression Process
Subjecting the coated paperboard to heated calender rolls pushes down the outstanding pigment and fiber peaks. This specular gloss elevation occurs as the micro-irregularities are flattened, transforming a diffuse-scattering surface into a highly reflective, mirror-like plane. Proper formulation of the coating, including the use of fine plate-like clay pigments, assists in this process by creating a naturally flatter starting surface.
However, the compression must be controlled to prevent the board from losing its thickness, which would reduce its bending stiffness.
Print Quality Effect
High gloss levels in the substrate enhance the brilliance and contrast of printed colors, especially when using transparent inks. Printers utilize these highly reflective boards to create premium cosmetic and beverage packaging that stands out on retail shelves. Monitoring the gloss with a multi-angle gloss meter ensures that the finish remains uniform across the entire production run.
Adjusting the ratio of latex binder to pigment in the coating formulation allows the mill to fine-tune the final reflective properties of the board.
