1. Synthesis of AKD
1.1 Preparation of Acid Chloride
The raw material for industrial AKD (Alkyl Ketene Dimer) used in papermaking is typically a mixture of industrial-grade saturated fatty acids. Acid chloride is produced by reacting fatty acids with thionyl chloride, as shown in the reaction below:
Since separating various organic acids is very difficult—a single fat source contains at least 5 different acids, and potentially up to 12 or more—AKD raw materials for papermaking are usually a mixture of stearic acid (C₁₈H₃₆O₂) homologs. The “1840” and “1865” wax powders commonly referred to in the industry indicate a 40% or 65% stearic acid content, respectively.
1.2 Preparation of Ketene
Acid chloride reacts with triethylamine to remove hydrogen chloride, forming an unstable ketene intermediate:
1.3 Intermolecular Lactone Ring Formation (Dimerization)
The unstable ketene undergoes intermolecular lactone ring condensation to form AKD molecules—hence the name “alkyl ketene dimer”:
The molecular formula of AKD can also be written as:
2. Emulsification of AKD
2.1 Preparation of Emulsifiers
At present, cationic starch and polymer emulsifiers are commonly used for AKD emulsification. Cationic starch is produced by the reaction of cassava starch and etherifying agent:
The main component of the polymer emulsifier is PolyDADMAC (Poly(dimethyl diallyl ammonium chloride)):
2.2 AKD Emulsification Process
AKD emulsification is mainly a physical dispersion process where the hydrophobic solid wax is uniformly dispersed in water to form a stable AKD emulsion.
Key steps of the emulsification process:
- Heat to melt AKD wax.
- Add emulsifiers.
- Apply high-shear mixing and homogenization.
- Cool the emulsion.
- Add additives such as aluminum sulfate and stabilizers.
The result is a positively charged AKD emulsion.
3. AKD Sizing
3.1 AKD Sizing Mechanism
Paper’s water resistance comes from the sizing reaction. As seen from the AKD molecular structure, it contains a hydrophobic alkyl chain and a hydrophilic lactone ring. Cellulose fibers contain many –OH groups and are naturally hydrophilic. Through reaction with AKD, hydrophobic groups are introduced to the fiber surface, imparting water resistance:
This reaction attaches one end of the AKD molecule to the fiber and arranges the hydrophobic alkyl chains outward, rendering the paper water-resistant.
3.2 AKD Sizing Process
The sizing process consists of three main stages:
- Adsorption: AKD particles adsorb onto the fiber surface, similar to filler retention. This occurs in the forming section.
- Melting and Spreading: In the drying section, AKD particles melt (above melting point) and spread over the fiber.
- Chemical Reaction: AKD reacts with the fiber –OH groups to form covalent ester bonds. This reaction requires time and is the essence of internal sizing.
4. Key Factors in Application
4.1 pH and Alkalinity Control
The sizing reaction depends on the lactone ring of the AKD molecule. The ring opens and reacts with –OH groups on fibers to form esters. The ring opens best in neutral to alkaline conditions, but excessive pH accelerates hydrolysis of AKD.
- Optimal pH for reaction: 6.0–8.0
- Recommended system pH: 7.5–8.5
- AKD emulsion storage pH: 2.5–3.5
Hydrolysis increases when pH exceeds 8, reducing sizing efficiency. Too low a pH (<6) prevents reaction but benefits AKD storage.
Alkalinity, expressed as buffering capacity, should remain between 50–200 ppm to maintain a stable system pH when adding acidic AKD emulsions.
4.2 Temperature Control
Temperature affects both AKD emulsion stability and reaction rate.
- AKD emulsions are solid-in-liquid suspensions, inherently thermodynamically unstable.
- Rising temperatures increase particle collisions and aggregation risk, causing solids to precipitate.
- Electrostatic repulsion from the cationic surface of AKD particles helps maintain stability.
In high-temperature environments, AKD particles may form skin layers or precipitate. However, this is not necessarily due to hydrolysis—since AKD hydrolysis is an exothermic reaction, rising temperatures may actually shift the equilibrium toward ester formation.
Storage temperature recommendation: 5–30°C
Paper machine system temperature: typically 45–50°C
AKD melting point: ~45–55°C
Drying section temperature: ≥60°C recommended for full reaction and spreading
To maximize retention and performance:
- Add AKD near the headbox to minimize melting in the approach system.
- Ensure the drying section provides adequate temperature and retention time.
5. Conclusion
AKD neutral sizing technology requires strict control over pH, alkalinity, and temperature at different stages. Mastery of these parameters ensures efficient sizing and high-quality paper. Other application challenges remain and warrant further industry-wide discussion and collaboration.
