Alkyl Ketene Dimer (AKD) sizing – a review
Alkyl Ketene Dimer (AKD) sizing – a review
Alkyl Ketene Dimer (AKD) sizing – a review
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Fig 3. a) Retention of cationic <strong>AKD</strong>-particles onto bleached kraft pulps using various cationic polyelectrolytes. b) Retention of anionic <strong>AKD</strong>-particles onto bleached kraft pulp<br />
using various cationic polyelectrolytes (Johansson and Lindström 2003b).<br />
fines and fillers follow the general retention level and<br />
there is little selective retention of certain types of dispersed<br />
material. Anionic dissolved substances in the stock,<br />
such as hemicelluloses and lignin residues, are generally<br />
detrimental to size retention (Lindström and Söderberg<br />
1986c).<br />
Spreading/size migration<br />
The distribution of <strong>AKD</strong>-size on the fibres occurs in the<br />
drying section as discussed above. <strong>AKD</strong> readily spreads<br />
on cellulose, because the cellulose surface is a high-energy<br />
surface. The free energy of spreading, ∆G s of <strong>AKD</strong> on<br />
a cellulose surface can be written:<br />
∆G s = γ(cellulose/<strong>AKD</strong>) + γ(<strong>AKD</strong>) - γ (cellulose) [1]<br />
The surface free energy of <strong>AKD</strong> is 27 mJ/m 2 (Garnier<br />
and Godbout 2000) and the surface free energy of dry<br />
cellulose has been determined to about 57 mJ/m 2 .<br />
(Lundqvist and Ödberg 1997; Luner and Oh 2001). If<br />
γ(cellulose/<strong>AKD</strong>) is small, the conditions for spreading,<br />
∆G s < 0, are fulfilled. For an <strong>AKD</strong> particle trapped in<br />
between a fibre-fibre bond the free energy of spreading,<br />
∆G s = 2γ(cellulose/<strong>AKD</strong>), which is a positive quantity,<br />
because it is associated with the cleavage of a high<br />
energy surface. Hence, <strong>AKD</strong>-particles trapped in between<br />
fibre-fibre bonds cannot spread and cannot react with<br />
cellulose. This phenomenon is the hypothesis for the<br />
limited reaction with cellulose (Lindström and O´Brian<br />
1986b). Spreading has been manifested by several investigators<br />
(Roberts and Garner 1985; Roberts et al. 1985;<br />
Ödberg et al. 1987; Seppänen et al. 2000; Horn 2001;<br />
Shchukarev et al. 2003). The spreading of <strong>AKD</strong> on<br />
cellulose should, however, not be associated with the<br />
common hydrodynamic phenomena of spreading<br />
(Cazabat 1989), which is a very rapid process. Instead, it<br />
has been suggested that spreading takes place by the<br />
surface diffusion of an autophobic monolayer of <strong>AKD</strong> on<br />
cellulose (Seppänen et al. 2000), which is a slower<br />
phenomena than hydrodynamic spreading. The apparent<br />
surface diffusion coefficient of <strong>AKD</strong> on cellulose have<br />
been calculated to around 10 -11 m 2 /s at 50-80°C<br />
204 Nordic Pulp and Paper Research Journal Vol 23 no. 2/2008<br />
(Seppänen et al. 2000; Shchukarev et al. 2003). As <strong>AKD</strong><strong>sizing</strong><br />
particles typically have the dimension of the order<br />
of a micrometer, such a droplet would on a cellulose<br />
surface spread within 10 sec, using this diffusion<br />
coefficient. The time of reaction is typically of the order<br />
of at least 5 minutes, hence spreading/surface diffusion is<br />
not the rate-determining step in <strong>sizing</strong>.<br />
More recent investigations have, however, challenged<br />
the traditional spreading view. Thus, Garnier et al<br />
(Garnier et al. 1998, 1999; Garnier and Godbout 2000)<br />
find that <strong>AKD</strong> only wets, but not spreads, on cellulose<br />
and claim vapour phase <strong>sizing</strong> as a <strong>sizing</strong> mechanism for<br />
<strong>AKD</strong>. Later investigations show vapour phase type of<br />
<strong>sizing</strong> with ASA but not with <strong>AKD</strong> at drying temperatures<br />
below 100°C. (Yu and Garnier 2002). A capillary<br />
wicking mechanism is also suggested by Garnier and<br />
Godbout (2000). One possibility is that the reservoir in<br />
the de Gennes type of experiments conducted by Garnier<br />
was simply too small. In this type of experiment a drop of<br />
the <strong>sizing</strong> agent is applied to a thread (cellulosic) and the<br />
contact angle is observed. A pre-requisite is that there is a<br />
sufficient surface area available for spreading to occur,<br />
otherwise spreading will stop when the available surface<br />
area is saturated with the monomolecular layer of the<br />
<strong>sizing</strong> agent, spreading stops and a finite contact angle is<br />
observed.<br />
Shen et al. also in a number of papers (Shen et al.<br />
2001a, 2001b; Shen and Parker 2003) claim that the<br />
classical view has been proven wrong and advocate<br />
mechanisms along the lines of Garnier and co-workers.<br />
These authors also claim there is no reaction between<br />
cellulose and <strong>AKD</strong>. Later investigations from this group<br />
(Hutton and Chen 2004), however, also show vapour<br />
phase <strong>sizing</strong> to be an insignificant phenomenon in<br />
practical papermaking <strong>sizing</strong>. The group has now (Shen<br />
and Parker 2003; Shen et al. 2005) adopted the autophobic<br />
precursor mechanism suggested by Seppänen et<br />
al. (2000).<br />
In our laboratory, a simple experiment was conducted<br />
to show that spreading does not take place in the gasphase<br />
and that spreading easily takes place and over<br />
macroscopic dimensions. Basically, two sets of experi-