Alkyl ketene dimers (AKDs) are a class of wax-like chemicals that have found widespread application in the papermaking industry. AKDs are primarily used as internal sizing agents to impart water resistance to paper. This allows paper products like magazines, catalogs, and posters to maintain their shape and print quality even when exposed to moisture.
Chemical Structure and Properties of AKDs
AKDs are dimerized ketene derivatives that are produced by the thermal decomposition of alkyl chloroformates. The generic chemical formula is CnH2n+1C=C=O where n is typically 12-22, representing a range of fatty acid-derived alkyl chains.
At room temperature, AKDs exist as colorless crystalline solids or waxes. They are non-volatile and hydrophobic in nature. In water, AKDs remain dispersed as colloidal solid particles rather than forming a true solution. Their melting points vary depending on the alkyl chain length, but are generally between 40-80°C.
Mechanism of Alkyl Ketene Dimer Sizing
When Alkyl Ketene Dimer are applied to paper during manufacture, the hydrophobic alkyl chains interact strongly with cellulose fibers through hydrogen bonding and physical adsorption. Upon drying, the AKD particles deposit onto the paper surface as a continuous, thin waxy coating.
This AKD layer prevents moisture from penetrating into the paper matrix. The ketene groups in the AKD structure allow for crosslinking reactions with cellulose hydroxyl groups and other AKD molecules, creating an insoluble and durable sizing barrier. Even after aging, the waxy film resists solubilization and maintains high sizing ability.
Application and Optimization in the Papermaking Process
Alkyl ketene dimer are most commonly added to the papermaking furnish during the wet end prior to sheet formation on the paper machine. Typical application levels range from 2-6 kg of AKD per ton of dry paper.
For optimal sizing performance, several process parameters must be controlled. The pH should be alkaline (>7) to ensure the AKD particles remain well dispersed. High shear mixing is also important to achieve uniform distribution across the furnish. Drying temperature, which influences AKD melting and infiltration, can impact sizing effectiveness. By finetuning these application variables, mills can customize AKD treatments for specific paper grades.
Advantages over Alternative Sizing Methods
Compared to alternative internal sizing techniques like alkyl succinic anhydride (ASA) and alkenyl succinic anhydride (AKA), AKDs offer certain benefits:
- Sizing effect persists through multiple recyclings without loss of performance. ASA/AKA sizing degrades with each repulping cycle.
- Imparts fast drying performance without increased energy costs. ASA requires higher drying temperatures that increase operating expenses.
- Ketene chemistry allows for greater bonding strength to cellulose fibers. alkyl ketene dimer films withstand mechanical abrasion better than ASA/AKA treatments.
- Produces a lower propensity for foam formation during papermaking. Excessive foaming reduces efficiencies.
- Compatible with fillers and coatings commonly used in paper manufacturing. Some sizing agents can detrimentally interact.
For the above reasons, AKDs remain the predominant commercial internal sizing technology employed by mills worldwide, especially for publications and specialty grades. Their robust and long-lasting performance helps paper withstand the demands of modern printing and converting operations.
alkyl ketene dimers leverage their unique chemical structure and interactions to provide durable water-resistance to paper products. As an internal sizing agent, AKDs deposit a crosslinked waxy coating within sheets to resist penetration and maintain integrity when exposed to moisture. Their advantages over other technologies, ease of application, and consistency have established AKDs as a fundamental component underpinning modern paper manufacturing.
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