Alkyl Ketene Dimer (AKD) sizing – a review

ments were conducted in additon to a reference sheet,
# 1 (Table 1). In the first experiment (# 2, Table 1) a sized
sheet was couched (after wet-pressing) between two
unsized sheets. This pack of sheets was then dried at
90ºC and the size retention and extent of reaction was
determined using radioactive labelled AKD as described
in our previous publications on AKD, e.g., Lindström and
Söderberg (1986a). After drying the stack of sheet was
simply delaminated and the total and reacted amount of
AKD could be determined. In the second experiment
(# 3, Table 1) a thin polytetrafluoroethylene (PTFE) wire
(allowing gas-phase transfer through the wire) was
placed between the sheets before drying.
These experiments showed two things: AKD could
spread across the whole stack of sheets, although the
mid-sheet still had the highest amount of AKD. In the
experiment using the PTFE wire, no transfer between the
sheets took place and consequently there was no gasphase
spreading across the pile of sheets.
Table 1.Results from AKD-transfer experiments. AKD-sized sheets (2.2 and 3.2)
were stacked between unsized sheets (sheets: 2.1, 2.3 and 3.1, 3.3) after wet
pressing and subsequently dried at 90ºC (series # 2) for 10 min. After drying, the
sheets were post cured at 110ºC for 10 min. The reference sheet was dried at
90ºC for 10 min and then at 110ºC for 10 min. Experiment # 3 was conducted as
2.1-2.3 but a thin polytetrafluoroethylene wire was inserted between the sheets
before couching to prevent size migration but allow for possible gas phase transfer
of AKD. 0.1% C-PAM (D.S. = 20 mole % cationic groups) was used as a
retention aid in the experiments. n.d. = not determined. Data not published previously.
Sheet Content AKD AKD AKD
in sample retention reacted amount
Exp. #/Sheet type mg/g % mg/g %
1.1 AKD (1.5 mg/g) 1.15 76.7 0.46 40.0
(Reference) +C-PAM
2.1 no AKD 0.16 0.07 43.8
2.2 AKD (1.5 mg/g) 0.88 80.0 0.38 43.2
+C-PAM
2.3 no AKD 0.16 0.06 37.5
3.1 no AKD [AKD] and the equation reads:
dx
dt
( )
= K [AKD] = K [AKD] = K a− x
[3]
av
1 2
where a is the maximum amount of reacted AKD and
consequently [AKD] av, the available amount for reaction
should be = a – x. Integration from t = 0 to t leads to:
ln a
a x Kt = [4]
−
Thus, if the quantity ln(a/(a – x)) is plotted versus time,
straight lines should be obtained and the reaction follows
what could be called a pseudo first order reaction. This is
illustrated in Fig 4a, which shows such graphs for the
reaction of AKD onto a bleached kraft pulp at different
pH-values. It is also shown that the straight lines intersect
the x-axis at a common point in time, independent of pH.
This is the time interval required for drying of the sheet
to a sufficiently high solids content for the AKD to begin
spreading. The spreading reaction as such is much faster
than the drying stage, which is independent of the pHvalue
as the rate of drying is independent on pH for this
type of pulp. If the same experiment then is performed at
different pH-values, the corresponding values between
the reaction rate constant K at different pH-values and
temperatures can be obtained and the Arrhenius activation
energies for the AKD/cellulose reaction can be
calculated from the graphs in Fig 4b.
The calculated activation energies for the reaction
between cellulose and AKD can be calculated from
Fig 4b to be 72 kJ/mole at pH 4 to 46 kJ/mole at pH 10
(Lindström and O´Brian 1986b). As expected the activa-
Fig 4. (a) The quantity In a/(a-x) versus reaction time, t, where a is the maximum
reacted AKD and x is the reacted amount of AKD at different pH-values. (b) The
reaction rate constant K, from Eq 2, versus 1/T, where T is the absolute temperature.
(Bleached softwood kraft pulp). (Lindström and O´Brian 1986b).
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