Sizing agents are applied into the paper to impart certain desirable qualities. The main function of sizing agent is to increase the resistance to penetration of water or other liquids into the paper so that the paper is suitable for printing, writing and other purpose. It can be achieved either by using wet-end additives or by applying a suitable coating to the surface of the dried paper. Sometimes a combination of treatments is required. The wet-end Sizing agents also decrease dusting, control spreads of inks, improve dewatering, retention of filler and fibers, improve paper quality, smoothness, dimension stability, abrasion, decrease porosity and improve paper machine runnability.
Types of Sizing Agent
There are two types of sizing agents; internal sizing agents and surface sizing agents.
Internal Sizing Agents
Internal sizing agents are used in papermaking process at the wet end. The most important internal sizing chemicals are alkyl ketene dimer (AKD), rosin and alkyl succinic anhydride (ASA). The internal sizing agents should have some basic characteristics such as high hydrophobicity, good retention on fibers, uniform distribution throughout the fiber surfaces, and must strongly bonded with fibers.
The internal sizing agents can also be classified into acidic type sizing chemical and basic or neutral type sizing chemical. Rosin and its derivatives are acidic type sizing agent. On the other hand AKD and ASA are basic or neutral type sizing agent.
Surface Sizing Agents
Surface sizing agents have both hydrophilic and hydrophobic ends. It is forms a thin film on paper web, the hydrophilic tail of it joined with the fiber, whereas hydrophobic tail facing outwards. The thin film reduces paper dust; improve surface strength, printability and water resistance of the paper. There are different chemicals are used as surface sizing agent such as modified starches, Styrene Maleic Anhydride (SMA), Styrene Acrylic Emulsion (SAE), Styrene Acrylic Acid (SAA), Ethylene Acrylic Acid (EAA), gelatin and Polyurethane (PUR). It is normally applied in a paper mill though the size press at dry end section.