Chemical Additive
Process of adding alkyl ketene dimer or alkenyl succinic anhydride to the pulp slurry to control the rate of liquid penetration into the finished paper or board. The use of internal sizing akd asa provides the substrate with resistance to water and ink without the need for a continuous surface film. These chemicals react with the cellulose fibres to create a hydrophobic surface throughout the entire thickness of the sheet.
Sizing Mechanism
Choice between these two chemicals depends on the specific requirements of the paper machine and the desired end use of the product. While alkyl ketene dimer reacts more slowly and is often used for board that requires long term moisture resistance, alkenyl succinic anhydride provides a rapid cure and is suited for high speed machines. Applying internal sizing akd asa prevents the feathering of ink and the excessive absorption of adhesives during the converting process.
These additives also help to maintain the dimensional stability of the paper when it is exposed to humid environments.
Wet End Chemistry
Efficiency of the sizing process is influenced by the pH of the system and the presence of other chemicals like calcium carbonate. Careful monitoring of the retention and the charge balance in the headbox ensures that the internal sizing akd asa particles stay on the fibres rather than being washed away. If the sizing is uneven, the board will show inconsistent print results and may fail in subsequent lamination or gluing steps.
Maintaining the chemical balance is a continuous requirement.
