Alkyl Ketene Dimer (AKD) sizing – a review

Fig 7a. The mechanism is probably
analogous to the mechanism of the
catalysis of the cellulose-AKD
reaction shown in Fig 7b, because
analysis of the reaction kinetics of the
reaction pointed to a trimolecular
reaction mechanism. In this investigation
it was also found that divalent
metal ions (Ca 2+ , Mg 2+ , Ba 2+ ) catalyzed
the hydrolysis reaction (see Fig 8a),
whereas the anion or monovalent electrolytes
had no effects. It is most likely
that the divalent cation functions
as a Lewis type of catalyst (Schinzer
1986), the mechanism of which is
shown in Fig 8b. Hydrolysis only
takes place at high temperatures and
during standard laboratory forming
and drying conditions hydrolysis is
insignificant.
The hydrolysis product of AKD, the
ketone in Fig 1, has a slight positive
effect on sizing, provided there is already
some reacted AKD present in the
paper (Lindström and Söderberg
1986a).
Amount of AKD required for sizing
The required amount of AKD for
sizing for a given pulp depends on a number of factors
and is also linked to a number of wet-end factors. Critical
is the retention of the size and the extent of reaction
together with the nature of the pulp furnish together with
the structure of the sheet.
Retention is critical, as recirculated size can be subject
to hydrolysis. The extent of reaction depends on the
drying conditions together with the presence of size
accelerators. The extent of reaction for AKD-sizes is
dependent on the fraction of fibre surface exposed to the
air phase, because it is only size on free surfaces, which
can spread and potentially react. AKD spreads all over
the sheet and sizing with AKD is not dependent on size
agglomeration, which is critical for instance with
soap/alum sizing.
The extent of reaction can be quite high under ideal
laboratory conditions, but under practical mill conditions
it is often in the range between 15-40%.
In an early publication (Lindström and Söderberg 1986a)
the amount required for sizing was investigated for
various extractive free pulps. Defining the onset of full
sizing as Cobb 60 = 25 g/m 2 , the required amount of AKD
necessary for sizing is directly proportional to the BET
surface area of the papers as shown in Fig 9. By using
surface balance measurements to determine the collapse
value of the monolayer the planar oriented monolayer
surface area of AKD can be calculated to 24 Å 2 per molecule.
It is important to emphasize that sizing is uniquely
defined by the reacted amount of AKD (Lindström and
Söderberg 1986a; Johansson and Lindström 2004b).
Using this surface area it can be calculated from Fig 9
Fig 7. (a) The effect of NaHCO 3 on AKD hydrolysis, (b) Suggested hydrolysis mechanism.
Fig 8. (a) The effect of CaCl 2 on AKD hydrolysis, (b) Suggested hydrolysis mechanism by Ca 2+ (Lewis acid catalysis).
(Lindström and Glad-Nordmark, 2007b).
Fig 9.The required amount of reacted AKD, necessary to obtain Cobb 60 = 25 g/m 2
for various pulps (extractives free).
that it is only necessary to cover 4% of the total surface
area for a given pulp in order to obtain sizing. In practice
the sweeping action of the fatty acid chains will, of course,
cover a much larger area. Ström and co-workers
(Ström et al. 1992) determined the required surface coverage
to 15% using ESCA. The electrons from carbon
atoms emitted in the ESCA analysis partly come from
carbon atoms underneath the fatty acid layer, so the surface
coverage values should not be directly compared.
Nordic Pulp and Paper Research Journal Vol 23 no. 2/2008 207