Alkyl Ketene Dimer

Chemical Action

Internal alkaline sizing agents provide paper and board with resistance to liquid penetration by altering the surface energy of the fibres. Alkyl ketene dimer achieves this by forming a covalent chemical bond with the hydroxyl groups of the cellulose molecules during the drying phase of the paper machine. This reaction establishes a hydrophobic barrier that prevents water-based liquids from soaking into the sheet.

Sizing efficiency depends on the temperature of the drying cylinders and the pH of the wet end, which should remain between seven and eight for optimal retention.

Application Window

Converting plants rely on the slow reaction profile of the chemical to manage subsequent processing steps. Alkyl ketene dimer requires up to twenty-four hours to cure fully and reach its maximum sizing value, meaning that initial off-machine tests often show lower resistance than the final matured product. This delayed curing allows for better adhesion in downstream coating and extrusion operations that occur immediately after manufacturing.

The curing rate can be accelerated by increasing the storage temperature of the reels.

Performance Barrier

Sizing limits are determined by the presence of calcium carbonate fillers and the specific requirements of the converting line. Excessive use of alkyl ketene dimer can lead to a slippery paper surface that interferes with high-speed winding and folding box gluing. High levels of the sizing agent reduce the coefficient of static friction, causing feed problems in packaging machines.