Internal Sizing

Sizing Agent

Processing involves adding sizing compounds such as alkyl ketene dimer or alkenyl succinic anhydride directly to the wet end of the paper machine. These molecules chemically bond to the cellulose fibers and create a hydrophobic barrier that resists liquid absorption. The amount of sizing added is carefully controlled to achieve the desired water absorption rate as measured by the Cobb test.

Insufficient sizing leads to soft and soggy board that fails during printing or storage.

Paper Grade

Consistent control of this chemical treatment prevents liquid penetration while maintaining the surface energy required for glue adhesion. The test stops applying to coated papers where a plastic barrier or heavy coating blocks all liquid contact. Recycled fiber stocks often require higher additions of sizing agents due to the presence of contaminants and shorter fiber lengths.

Maintaining the correct pH in the headbox is essential for the sizing molecules to bond effectively.