from Wikipedia "Surface sizing solutions consists of mainly modified
starches and sometimes other hydrocolloids, such as
gelatine, or surface sizing agents such as
alkyl ketene dimer (AKD) or
acrylic co-polymers. Surface sizing agents are
amphipathic molecules, having both
hydrophilic (water-loving) and
hydrophobic (water-repelling) ends. The sizing agent adheres to substrate fibers and forms a film, with the hydrophilic tail facing the fiber and the hydrophobic tail facing outwards, resulting in a smooth finish that tends to be water-repellent. Sizing improves the surface strength, printability, and water resistance of the paper or material to which it is applied. In the sizing solution,
optical brightening agents (OBA) may also be added to improve the opacity and whiteness of the paper or material surface."
aren't you glad you asked
