The invention provides a low density paperboard material and associated
method for use in producing an insulated container, and is especially
well-suited for making cups. The paperboard material comprises a
paperboard web including wood fibers and expanded microspheres, and has a
relatively low density ranging from about 6 to about 10 lb/3 MSF/mil, a
relatively high caliper ranging from about 24 to about 35 mil, and an
internal bond strength of at least about 80.times.10.sup.-3 ft-lbf.,
preferably at least 100.times.10.sup.-3 lft-lbf. For applications such as
cups the material is also coated on one or both sides with a barrier
coating, preferably low density polyethylene, to limit liquid penetration
into the web. The low density paperboard material of the invention is
convertible for manufacture of containers, particularly cups, and
exhibits insulative properties comparable to higher cost materials
conventionally used to make cups. Also, the surface of the low density
board may have a Sheffield smoothness of 300 SU or greater compared with
the surface smoothness of 160 to 200 SU for conventional cupstock, the
latter having been thought necessary for adequate print quality. However,
it has been found that the low density board exhibits good printability
on flexo printing machines despite its relatively rough surface, which is
surprising and bonus effect realized along with the insulative and other
properties of the board.