Sizing Agent: Types, Use, and Starch-based

Sizing agent is a substance that is used in the papermaking process to increase the water-resistance of the paper’s surface. The primary function of a sizing agent is to prevent ink or other liquids from penetrating the paper fibers and causing smudging, blurring, or other undesirable effects. Sizing agents can be added to the paper pulp during the manufacturing process or applied to the surface of the paper after it has been formed. There are two main types of sizing agents: internal sizing agents that are added to the pulp and surface sizing agents that are applied to the paper surface. Common types of sizing agents include alkyl ketene dimers, rosin, modified starches, and polyurethane.

Types of sizing agent

There are two main types of sizing agents: internal sizing agents and surface sizing agents.

  1. Internal Sizing Agents: These agents are added to the paper pulp during the papermaking process at the wet end. They penetrate the fibers and make them hydrophobic (water-resistant) to prevent water or other liquids from penetrating the paper. Common examples of internal sizing agents include alkyl ketene dimer (AKD), rosin, and alkyl succinic anhydride (ASA).
  2. Surface Sizing Agents: These agents are applied to the surface of the paper after it has been dried. They form a thin film on the paper’s surface to provide water-resistance and improve its printability, strength, and other properties. Surface sizing agents can be applied using a size press or other coating methods. Examples of surface sizing agents include modified starches, Styrene Maleic Anhydride (SMA), Styrene Acrylic Emulsion (SAE), Styrene Acrylic Acid (SAA), Ethylene Acrylic Acid (EAA), gelatin, and Polyurethane (PUR).


Sizing agents are used in papermaking to impart desirable qualities to paper, such as increased resistance to penetration of water or other liquids. The main use of sizing agent is to make paper suitable for printing, writing, and other purposes. The benefits of sizing agents include:

  1. Water resistance: Sizing agents make paper water-resistant, preventing ink from bleeding and blurring due to contact with water.
  2. Improved printability: Sizing agents improve printability by preventing ink from spreading, resulting in sharper and clearer prints.
  3. Reduced dusting: Sizing agents decrease dusting, which is important for machine runnability and end product quality.
  4. Improved surface strength: Sizing agents improve surface strength, making paper more resistant to tearing and other damage.
  5. Enhanced dimensional stability: Sizing agents improve dimensional stability, preventing paper from shrinking or expanding excessively in response to changes in temperature and humidity.
  6. Better dewatering and retention: Sizing agents improve dewatering and retention of fibers and fillers, leading to better paper quality and improved production efficiency.

Modified starch as sizing agent

Modified starch can be used as a surface sizing agent in papermaking. The modified starch can improve the surface strength, printability, and water resistance of the paper. Additionally, it can reduce paper dust and improve the paper’s dimensional stability and runnability on the paper machine. Modified starch is often used as an alternative to natural starches because of its improved performance properties, such as better stability and consistency, higher viscosity, and resistance to high temperatures and shear forces. However, the use of modified starch as a sizing agent can be limited by its higher cost compared to other sizing agents and challenges in achieving a homogeneous product.

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