Alkyl ketene dimer (AKD) is most common neutral/ alkaline sizing agent. It is an Internal sizing agents. Structurally, it is unsaturated lactones and it is synthesized from fatty acids. Physically it is waxy like milky emulsion chemicals. The waxy solid tiny particles (0.5–2 mm) are dispersed into water and the emulsification are stabilized by the cationic starch or another cationic polyelectrolyte stabilizer.
The reactive lactones rings of AKD can easily goes under chemical reaction. It reacts with the hydroxyl groups of cellulose and forming an ester bond (b-keto esters).
The availability and the reasonably priced of calcium carbonate makes it the popular filler material. But this filler material is unstable in acidic medium, hence the usage of neutral/alkaline sizing agent are increased.
Recent years AKD is the most extensively used reactive sizing agent in the high quality printing papers production.
Alkaline sizing agent improves surface conditions of paper. The Neutral/Alkaline Sizing Agent (AKD) also reduces paper machine breakdown and water recycling difficulties compare to Acidic sizing agent.
The function of AKD sizing agent depends on different technological parameters such as ph value of the fiber suspension, raw materials, method of drying, temperature, time etc. AKD is much less reactive than ASA; hence it is more flexible for papermakers during usages. It means that it can be add to the thick or thin stock. However the presence of PCC in the system can lessen the efficiency of the sizing agent.
It should be followed the manufacturers instruction during storage especially temperature and time since AKD is slowly hydrolyzed.