Internal Treatment
Controlling the absorption of water and inks into the fibrous matrix is a fundamental requirement of the papermaking process. The application of wet end sizing introduces hydrophobic chemicals directly into the pulp slurry before the paper sheet is formed on the wire. This internal treatment ensures that the liquid resistance is distributed evenly throughout the entire thickness of the finished paper.
Chemical Retention
Synthetic sizing agents such as alkyl ketene dimer or alkenyl succinic anhydride are added to the fiber suspension in the approach flow. During wet end sizing, these chemical compounds must attach to the negatively charged pulp fibers before sheet formation. A cationic polymer retention aid is often required to bind the emulsified sizing particles to the cellulose surfaces.
Once retained, the heat of the paper machine dryers activates the sizing agent, causing it to react chemically with the hydroxyl groups of the wood pulp. This chemical reaction creates a permanent non-polar barrier that resists liquid penetration.
Absorbency Control
Unsized paper acts like a blotting sheet, absorbing liquids rapidly through capillary action. Implementing wet end sizing reduces this capillary force, allowing the paper to be used for printing or liquid packaging. This control of absorbency prevents feathering of inks and maintains the structural strength of the paper when exposed to moisture.
