Alkyl Ketene Dimer (AKD) sizing – a review

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It is important to avoid surface active substances in the dispersion formulation, because they may interfere with sizing (see below). The dispersions are usually made slightly cationic in order for them to have a natural substantivity to negatively charged fibres, but anionic dispersions are also used in commercial practice. In order to avoid hydrolysis of the AKD, the pH is kept around 3 in the formulations. There have been some recent studies on the stability of AKD-dispersions. It has been found that the stability increases by the presence of AKD-oligomers (Asakura et al. 2006a) and that fatty acid anhydrides (byproduct in the AKD-dispersion) decrease the heat shock stability (Asakura et al. 2006b). The effect of various colloidal substances present in process waters have also been investigated (Mattsson et al. 2001). AKD dispersions basically behave as electrostatically stabilized colloids. Consecutive events of AKD-sizing The consecutive events in AKD-sizing are (Lindström and Söderberg 1986a): · Retaining the AKD-size using appropriate retention strategies for the size · Spreading/size migration to a monolayer · Chemical reaction of the size with the cellulosic fibres The retention mechanism is, in theory, heterocoagulation, where cationic size particles are attached to the negatively charged fibres. This is expected to give a good distribution of the dispersion particles, but practice shows that size distribution is not critical, because of extensive spreading on the fibre surfaces. More important is that effective retention aids must be used for the purpose. Heterocoagulation, cannot be used to retain AKD-particles under high fibre-fibre shear conditions. A high single pass retention is important, because recirculated size is hydrolyzed in the white water of a paper machine. When the size particles have been deposited on the fibres, the reaction with cellulose is insignificant because very few molecules are in molecular contact with cellulose. Because of the high surface tension of water, no spreading can take place until the water has been removed and the size particles are in direct contact with air. Hence, an air-AKD surface must be formed before spreading can take place and this takes place during drying at a solids content exceeding 60%. The spreading continues until a monomolecular layer has been formed. This layer then reacts with the hydroxyl groups of cellulose. The reaction is slow at low pH-values and, in practice; AKD cannot be used except in the neutral or slightly alkaline pH-range. Moreover, sizing accelerators (most commonly HCO - 3) are almost always required for effective development of sizing. Retention of AKD Cationic AKD-particles are theoretically retained by a heterocoagulation deposition mechanism onto the negatively charged fibres, but studies (Johansson and Lindström 2004b) have shown that the electrostatic attraction is screened by electrolyte concentrations commonly present in process waters, so retention aids must always be used in commercial practice. Secondly, the deposition is a highly dynamic process, where AKDparticles are rapidly deposited, after which they are sheared off the fibre surfaces (Lindström and Söderberg 1986c; Champ and Ettl 2004). The surface charge of fibres is important for the selfretention (retention of AKD without retention aid addition) of AKD, as self-retention is a heterocoagulation mechanism (Isogai et al. 1997; Lindström and Glad- Nordmark 2007a). There is an optimum surface charge density of fibres and an optimum electrolyte concentration for maximum AKD-retention (Lindström and Glad- Nordmark 2007a). These studies are, however, laboratory studies, and in commercial practice self-retention is most likely negligible due to high electrolyte concentrations and the high shear to which the fibre suspension is subjected. The retention of AKD by cationic polyelectrolytes has been studied by several other groups (Esser and Ettl 1997; Hasegawa et al. 1997; Isogai 1997a, 1997b). It is not self-evident that it should be possible to retain cationic AKD-dispersions using cationic polyelectrolytes. Due to the fact that AKD-dispersions usually are dispersed using a combination of lignosulfonates and cationic starch, the net cationically charged dispersion particles have in fact an amphoteric nature. Hence cationic polyelectrolytes can interact with the negative sites on the dispersion particles and retain the AKD-particles. As shown in Fig 3, anionic AKD-particles are easier to retain by cationic polyelectrolytes than cationic AKDparticles (Johansson and Lindström 2004b). The appropriate choice of an efficient retention aid system is crucial, because AKD will be subject to hydrolysis when circulated in the short circulation of a paper mill, as will be discussed below. The use of hydrophobic retention aids has also been practised (Riebeling et al. 1996, 1999) to treat filler so that the interaction between filler particles and AKD could be minimized. Obviously AKD cannot react with fillers and CaCO 3-based fillers promote hydrolysis (see below). Pre-flocculation of AKD leads to higher AKD-retention (Mattsson et al. 2002), which in itself is beneficial to sizing. Agglomeration is not critical to sizing (Johansson and Lindström 2004b); neither is the particle size (Petander et al. 1998) because AKD spreads over the fibre surfaces anyhow. In the practical use of AKD, the point of addition is of course dependent on the wet end system and the addition order of other chemical adjuvants. As a rule of thumb it is advantageous to add AKD to the high consistency stock only a short time before dilution takes place in the short circulation. The AKD-particles are deposited at a rapid rate at high consistency but are subsequently sheared off the fibre surfaces by the other fibres in a stirred pulp suspension (Lindström and Söderberg 1986a; Champ and Ettl 2004). Both high shear and long contact times are known to reduce AKD retention. As a rule, both AKD, Nordic Pulp and Paper Research Journal Vol 23 no. 2/2008 203
Fig 3. a) Retention of cationic AKD-particles onto bleached kraft pulps using various cationic polyelectrolytes. b) Retention of anionic AKD-particles onto bleached kraft pulp using various cationic polyelectrolytes (Johansson and Lindström 2003b). fines and fillers follow the general retention level and there is little selective retention of certain types of dispersed material. Anionic dissolved substances in the stock, such as hemicelluloses and lignin residues, are generally detrimental to size retention (Lindström and Söderberg 1986c). Spreading/size migration The distribution of AKD-size on the fibres occurs in the drying section as discussed above. AKD readily spreads on cellulose, because the cellulose surface is a high-energy surface. The free energy of spreading, ∆G s of AKD on a cellulose surface can be written: ∆G s = γ(cellulose/AKD) + γ(AKD) - γ (cellulose) [1] The surface free energy of AKD is 27 mJ/m 2 (Garnier and Godbout 2000) and the surface free energy of dry cellulose has been determined to about 57 mJ/m 2 . (Lundqvist and Ödberg 1997; Luner and Oh 2001). If γ(cellulose/AKD) is small, the conditions for spreading, ∆G s < 0, are fulfilled. For an AKD particle trapped in between a fibre-fibre bond the free energy of spreading, ∆G s = 2γ(cellulose/AKD), which is a positive quantity, because it is associated with the cleavage of a high energy surface. Hence, AKD-particles trapped in between fibre-fibre bonds cannot spread and cannot react with cellulose. This phenomenon is the hypothesis for the limited reaction with cellulose (Lindström and O´Brian 1986b). Spreading has been manifested by several investigators (Roberts and Garner 1985; Roberts et al. 1985; Ödberg et al. 1987; Seppänen et al. 2000; Horn 2001; Shchukarev et al. 2003). The spreading of AKD on cellulose should, however, not be associated with the common hydrodynamic phenomena of spreading (Cazabat 1989), which is a very rapid process. Instead, it has been suggested that spreading takes place by the surface diffusion of an autophobic monolayer of AKD on cellulose (Seppänen et al. 2000), which is a slower phenomena than hydrodynamic spreading. The apparent surface diffusion coefficient of AKD on cellulose have been calculated to around 10 -11 m 2 /s at 50-80°C 204 Nordic Pulp and Paper Research Journal Vol 23 no. 2/2008 (Seppänen et al. 2000; Shchukarev et al. 2003). As AKDsizing particles typically have the dimension of the order of a micrometer, such a droplet would on a cellulose surface spread within 10 sec, using this diffusion coefficient. The time of reaction is typically of the order of at least 5 minutes, hence spreading/surface diffusion is not the rate-determining step in sizing. More recent investigations have, however, challenged the traditional spreading view. Thus, Garnier et al (Garnier et al. 1998, 1999; Garnier and Godbout 2000) find that AKD only wets, but not spreads, on cellulose and claim vapour phase sizing as a sizing mechanism for AKD. Later investigations show vapour phase type of sizing with ASA but not with AKD at drying temperatures below 100°C. (Yu and Garnier 2002). A capillary wicking mechanism is also suggested by Garnier and Godbout (2000). One possibility is that the reservoir in the de Gennes type of experiments conducted by Garnier was simply too small. In this type of experiment a drop of the sizing agent is applied to a thread (cellulosic) and the contact angle is observed. A pre-requisite is that there is a sufficient surface area available for spreading to occur, otherwise spreading will stop when the available surface area is saturated with the monomolecular layer of the sizing agent, spreading stops and a finite contact angle is observed. Shen et al. also in a number of papers (Shen et al. 2001a, 2001b; Shen and Parker 2003) claim that the classical view has been proven wrong and advocate mechanisms along the lines of Garnier and co-workers. These authors also claim there is no reaction between cellulose and AKD. Later investigations from this group (Hutton and Chen 2004), however, also show vapour phase sizing to be an insignificant phenomenon in practical papermaking sizing. The group has now (Shen and Parker 2003; Shen et al. 2005) adopted the autophobic precursor mechanism suggested by Seppänen et al. (2000). In our laboratory, a simple experiment was conducted to show that spreading does not take place in the gasphase and that spreading easily takes place and over macroscopic dimensions. Basically, two sets of experi-
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Alkyl Ketene Dimer (AKD) sizing – a review

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