ADHESION PHENOMENA IN (CO)EXTRUSION COATING OF PAPER AND PAPERBOARD

In extrusion coating, the inadequate adhesion between the polymer coat ing and the fiber-based paper substrate (paper and paperboard) is both a common and a constant problem. The lack of adhesion between the pri nting ink, or glue, and the polymer coating is another area where adhe sion improvement is needed. The common means of improving adhesion are flame, corona, and ozone treatments. A modern extrusion coating line is equipped with both a pretreatment and a post-treatment unit. From t he work presented here, the following observations were made. The high er the applied corona power and the thicker the coating, the higher th e surface energy and polarity of the low density polyethylene (PE-LD) surface. When a high corona power was applied to the coating, only the polar component of the surface energy was increased. The surface ener gy decreased sharply as a function of aging, but remained more or less constant after about 2 weeks' storage time. The contact angles of wat er on paper correlated well with the oxygen contents (determined by ES CA) and with the applied corona power. The polarities of both paper an d paperboard increased as a function of the applied corona power. Coro na pretreatment of paper and paperboard improved their adhesion to PE- LD remarkably. The adhesion of the polypropylene (PP) homopolymer is b ased more on mechanical interlocking than on interfacial bonding. On t he other hand, the oxidizing pretreatments of the paper substrates sig nificantly promoted the adhesion of the PP copolymer.