Sizing is a critical process in paper manufacturing that enhances paper functionality by improving its water resistance, surface smoothness, and printability. The two primary methods—internal sizing and surface sizing—can specifically target and refine paper properties, making them essential for meeting the diverse demands of various paper grades.

Internal sizing agents are added to the pulp slurry before paper formation. They adsorb onto cellulose fibers and coalesce during the drying process to form a hydrophobic barrier within the fiber network. This barrier reduces moisture penetration into the paper interior, ensuring the paper maintains its structural integrity in humid environments.

Common Types of Internal Sizing Agents：

Alkyl Ketene Dimer (AKD): Renowned for its long-term stability, it is suitable for printing/writing papers requiring consistent waterproof performance.

Alkenyl Succinic Anhydride (ASA): Features rapid reaction speed, making it ideal for high-speed paper machines and delivering excellent initial waterproofness.

Rosin Sizing Agent: A traditional and cost-effective option, it is primarily used for newsprint and provides basic waterproof performance.

Internal sizing establishes the foundation for the overall water resistance of paper, preventing fiber swelling caused by moisture and avoiding paper damage due to dampness.

Surface sizing agents are applied to the dried paper web through the size press. They fill surface pores, deposit a thin protective film, and enhance surface properties such as strength, smoothness, and printability.

Typical formulations include:

Oxidized starch: Enhances surface smoothness for offset printing papers, ensuring clear ink transfer.

Cationic starch: Improves surface strength and ink adhesion, which is crucial for packaging papers.

Polyvinyl alcohol (PVA): Provides excellent water/oil resistance, suitable for food-grade packaging.

Styrene-acrylic emulsion: Balances water resistance with tactile feel, used for specialty papers.

Surface sizing addresses specific surface requirements—such as print clarity for printing papers or oil resistance for food packaging papers—where external performance demands are highest.

The synergistic effect between the two

Internal sizing and surface sizing typically work synergistically. For example:

Combining AKD (internal sizing for long-term water resistance) with cationic starch (surface sizing for printability) creates balanced office paper.

Rosin (internal sizing) paired with PVA (surface sizing) produces corrugated cardboard with compressive strength and moisture resistance.

This synergy ensures paper meets stringent standards—from art paper requiring vibrant color reproduction to medical packaging paper demanding sterility and waterproofness.

 Internal sizing establishes the foundational resistance to compression, while surface sizing optimizes surface functionality. The combination of both enables manufacturers to tailor paper properties to specific applications, striking an optimal balance between performance, cost, and sustainability. Mastering these sizing techniques is key to producing paper that excels in its intended use—whether for printing, packaging, or special-purpose applications.