Alkyl Ketene Dimer (AKD) sizing – a review

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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). Nordic Pulp and Paper Research Journal Vol 23 no. 2/2008 205
tion energy decreases for the nucleophilic reaction the higher the pH. Sizing accelerators The reaction between AKD and cellulose is, however slow and sizing accelerators are invariably used in commercial operations, whereby the reaction rate easily can be increased 20 times. The most important sizing accelerators are: · HCO-3 · Basic polymers with amine groups The HCO - 3-ion has a unique ability to catalyse the reaction between AKD and cellulose. HCO - 3- -ions are often inherently present in natural systems, e.g. when CaCO3 is used as filler, but it is also a general practice to add NaHCO3 to increase the alkalinity of the stock. Fig 5a shows how the reaction of AKD in the presence of NaHCO3 is catalysed. The acceleration has been quantified using the above reaction rate expression. A further analysis of the so obtained reaction rate constant reveals that the reaction rate is proportional also to [HCO - 3]. Hence, the reaction follows the equation: dx dt K - = [cellulose] ⋅[AKD] ⋅[HCO ] 0 3 This equation clearly suggests that there is a trimolecular reaction between cellulose, AKD and HCO 3 taking place. The suggested mechanism of catalysis is given in Fig 5b. Polymeric amines (e.g. PAMAM-EPI resins) having amino groups with a free electron pair are classic sizing accelerators for AKD (Lindström and Söderberg 1986d; Thorn et al. 1993; Cooper et al. 1995). Several different Fig 5. (a) The quantity ln a/(a-x) versus reaction time, t, where a is the maximum reacted AKD and x is the reacted amount of AKD at different bulk concentrations of NaHCO 3. (b) A suggested mechanism of catalysis with NaHCO 3. (Lindström and Söderberg 1986d). 206 Nordic Pulp and Paper Research Journal Vol 23 no. 2/2008 [5] Fig 6. Synergistic effects between HCO - 3and PAMAM-EPI resin on the reaction rate constant, K, when simultaneously used in AKD-sizing. ❍ = sizing in deionized water at different pH-values. ●– = sizing in the presence of 0.1% PAMAM in deionized water (Lindström and Söderberg 1986a). types of condensation polymers have been investigated over the years and occur in commercial formulations. The polymeric sizing accelerators are often either added to the AKD-dispersion (”rapid curing dispersion”) or used separately as combined accelerator and retention aid. Moreover the effects of HCO - 3 are synergistic, as shown in Fig 6. AKD-hydrolysis AKD can also react with water forming the β-keto acid, which spontaneously decarboxylates forming the corresponding ketone, as shown in Fig 1. AKD is, however, stable at room temperature at acidic pH-values, allowing storage at the time scale of months. It is well known that AKD can be the subject to alkaline hydrolysis. It is known that CaCO 3 may induce hydrolysis, particularly precipitated calcium carbonates (PCC) having higher pH-values due to residual alkali (Colasurdo and Thorn 1992; Novak and Rende 1993; Bottorff 1994; Jiang and Deng 2000). As AKD is strongly adsorbed onto PCC and gronud calcium carbonate (GCC), it is advantageous to preadsorb cationic polymers onto carbonate fillers in order to block AKDdeposition (Esser and Ettl 1997). There have, however, been few systematic and quantitative investigations in this field. In a recent investigation from this lab, the hydrolysis was studied using 14 C-labelled AKD (Lindström and Glad-Nordmark 2007b). It was found that NaHCO 3 catalyzed the hydrolysis reaction, as shown in
Page 1 and 2: Alkyl Ketene Dimer (AKD) sizing - a
Page 3: Fig 3. a) Retention of cationic AKD
Page 7 and 8: Practical aspects and comparisons b

Alkyl Ketene Dimer (AKD) sizing – a review

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