Water resistance improvement of paperboard

The research assessed how type, particle size, and concentration of pigment in coating formulations influenced water and water vapor barriers of paperboard packaging.

Calcium carbonate (CaCO3) and kaolin of micro- and nanosize of 1 and 0.05 µm, respectively, were added to 3% (w/v) gelatinised cassava starch to prepare coating formulations with different pigment concentrations of 5, 15, and 25% (w/v). These were applied on paperboard surfaces using the rod coating technique. Results showed that water resistance of paperboard, evaluated by contact angle (CA) and water absorptiveness (Cobb) were strongly influenced by pigment type (chemistry), particle size and shape, and pigment concentration (coating layer thickness).

A hydrophobic surface (CA > 100) was obtained only for the nano-CaCO3 formulation with 25% (w/v) by combined effects of the nanosize scale, the spherical shape, and the hydrophobic surface of CaCO3 particulates. Kaolin nanoparticle coatings produced hydrophilic surfaces (CA ~ 90) with high water absorptiveness, while high concentrations of kaolin microparticle coating slightly improved water and water vapor barriers by compact arrangement of flattened particles.

Mechanical strength properties of the paperboard were not significantly affected by coatings, but surface cracking was observed for large particle size or at high pigment concentration due to the increase in pigment/starch ratio.