Internal sizing agents are typically added to the pulp during the beating or stock preparation stage before paper formation. Internal sizing can endow paper and board with properties such as ink resistance, water resistance, emulsion resistance, and corrosion resistance, thereby improving smoothness, water resistance, and printing adaptability.
Aluminum sulfate has both hydrophilic and hydrophobic groups, which can be uniformly retained on the fiber surface by itself or with the help of other retention aids. The hydrophilic groups bind to the fibers, while the hydrophobic groups face the outer side of the fibers, thereby reducing the surface free energy between the fibers and air. This change in surface free energy alters the contact angle of liquids on the fiber surface, achieving the purpose of sizing. Aluminum sulfate can also adjust the pH value of the surface sizing solution. To ensure the effectiveness of anionic surface sizing agents, the surface sizing solution must be maintained in an acidic condition.
Except for special paper types such as blotting paper, filter paper, wax paper, cigarette paper, and household paper, almost all papers require sizing, making aluminum sulfate widely used in the papermaking field.