Internal sizing agents are typically added to the pulp during the pulping or stock preparation stages, before the paper sheet formation. Internal sizing imparts properties such as resistance to ink, water, emulsion, and corrosion to the paper and board, enhancing smoothness, water resistance, and printability.
Alumina sulfate possesses both hydrophilic and hydrophobic groups, which can uniformly retain on the fiber surface either by itself or with the assistance of other retention agents. This allows the hydrophilic groups to bind with the fibers, while the hydrophobic groups turn towards the fiber exterior, thereby reducing the surface free energy between the fibers and air, altering the contact angle of the liquid on the fiber surface, and achieving the goal of sizing. Additionally, alumina sulfate can adjust the pH value of the sizing liquid; to ensure its effectiveness, the sizing liquid must maintain an acidic, anionic surface sizing agent.
In addition to absorbent papers, filter papers, wax papers, cigarette papers, and domestic paper products, almost all types of paper require sizing. Aluminum sulfate has a wide application in the papermaking industry.
