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Sizing is a substance that is applied to fibers during paper manufacture in order to curb their tendency to absorb liquids by capillary action. By doing so, sizing keeps the ink on the surface of the paper where it was intended to remain. In addition, sizing affects abrasiveness, creasibility, finish, printability, smoothness, and surface bond strength, and decreases surface porosity and fuzzing.
There are two major types of sizing: engine (rosin) and surface (tub). Rosin sizing is applied to almost all papers and especially to all those that are machine made, while tub sizing is added for the highest grade bond, ledger, and writing papers. Tub sizing consists of gelatin glue and / or starch and is generally only used for handmade papers. Rosin is an amphipathic molecule, having both hydrophilic (water-loving) and hydrophobic (water-repelling) ends. The rosin coats the paper fiber and forms a film, with the hydrophilic tail facing the fiber and the hydrophobic tail facing outwards. This creates a water-repellent situation, which causes the water-based ink to remain outside on the paper surface.
There are three categories of papers with respect to sizing: unsized (water-leaf), weak sized (slack sized), and strong sized (hard sized). Waterleaf has low water resistance and includes absorbent papers for blotting. Slack sized paper is somewhat absorbent and includes newsprint, while hard sized papers have the highest water resistance.
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