US Patent 7749583 Low density paperboard

A paperboard including a solid bleached sulfate paperboard substrate and a coating applied to the paperboard substrate to form a coated structure, the coated structure having a basis weight, a caliper thickness and a Parker Print Surf smoothness, the Parker Print Surf smoothness being at most about 3 microns, the basis weight being at most about Y1 pounds per 3000 ft2, wherein Y1 is a function of the caliper thickness (X) in points and is calculated as follows: Y1=3.79+13.43X−0.1638X2.