Paper sizing is an essential operation in the paper industry that involves treating the paper with various substances to improve its properties. The main purpose of sizing is to control the absorbency of the paper, making it suitable for different applications such as writing, printing, or packaging.

The sizing process can be divided into two categories: internal and surface sizing. Internal sizing involves adding chemicals directly into the pulp during the papermaking process. Commonly used agents for internal sizing include rosin and alkyl ketene dimer (AKD). These substances form a hydrophobic layer on the fibers, reducing their ability to absorb liquids.

Surface sizing, on the other hand, is applied after the paper has been formed. It involves coating the surface of the paper with a thin layer of sizing agent using a size press or a film press. Surface-sizing agents can be natural or synthetic polymers that enhance water resistance and improve printability.

The benefits of this operations are numerous. Sized papers have improved ink holdout, preventing ink from feathering or bleeding through. They also exhibit better dimensional stability and reduced curling when exposed to moisture. Sizing enhances strength properties such as tear resistance and tensile strength while maintaining flexibility.

In conclusion, paper sizing is a crucial operation in the paper industry that significantly impacts the quality and performance of papers. By controlling absorbency and enhancing various properties, sized papers become more versatile for different applications in writing, printing, and packaging industries.