Using simple, joint-strength measurements, the bond between printing papers and various thermoplastic-polymer formulations is defined. It is demonstrated that the bond strength to the paper-polymer interface strongly depends on the sizing processes with which the paper has been treated during manufacturing. Increased amounts of rosin sizes and synthetic, cellulose reactive sizing agents both lower the paper-polymer adhesion. However, the effect is more pronounced for synthetic-sizing agents, as also demonstrated by Swanson and others for extrusion-coated papers and board. The decrease in bonding strength correlates with a decrease in paper wettability, as measured by capillary penetration of organic liquids of suitable surface tension, or as defined by surface energetics using the elution gas-chromatography technique.