a study of akd-size retention, reaction and sizing … report 162.pdfaccording to...

A Study of AKD-Size Retention, Reaction and Sizing Efficiency

Part 4: Mechanisms of AKD-hydrolysis

Tom Lindström and Gunborg Glad Nordmark

May 2006

According to Innventia Confidentiality Policy this report is public since June 2007

a report from STFI-Packforsk

A Study of AKD-Size Retention, Reaction and Sizing Efficiency

Part 4: Mechanisms of AKD-hydrolysis

Tom Lindström and Gunborg Glad-Nordmark

STFI-Packforsk report no.: 162 | May 2006

Cluster: Paper Chemistry Restricted distribution to:

Billerud, Eka Chemicals, Holmen, Kemira, Kornäs, Mondi Packaging Paper, M-real, Norske Skog, Stora Enso, Södra, Voith

According to STFI-Packforsk's Confidentiality Policy this report is assigned category 2

A Study of AKD-Size Retention, Reaction and Sizing Efficiency STFI-Packforsk Cluster Report

2 (24)

Acknowledgements The financial contribution from Billerud, Eka Chemicals, Holmen Paper, Kemira Oy, Korsnäs, Mondi Packaging Paper, M-real, Norske Skog, Stora Enso, Södra, Voith and the Swedish Energy Agency is greatly acknowledged.




3 (24)

Table of contents Page

1 Summary.......................................................................................................4

2 Introduction ..................................................................................................6

3 Materials manufacture.................................................................................7

3.1 Preparation of C-14-labelled AKD-dispersion.....................................7

3.2 Determination of AKD-hydrolysis ........................................................7

4 Results and Discussion ..............................................................................8

5 Conclusions ...............................................................................................20

6 References..................................................................................................21




4 (24)

1 Summary Conditions for the hydrolysis of an Alkyl Ketene Dimer (AKD) dispersion was investigated. The investigated AKD dispersion was a C-14 labelled AKD, which on hydrolysis degrades to the corresponding ketone, releasing C14O2. By measuring residual activity in the dispersion, the extent of hydrolysis could be quantified. The kinetics of AKD-hydrolysis was investigated under different temperatures, pH-condi-tions and chemical environments (electrolytes). As expected hydrolysis was faster at a higher pH.

More important, it was found that the presence of HCO3- and divalent metal ions

(Ca2+, Ba2+ and Mg2+) catalyzed the AKD-hydrolysis reaction. The catalysis of these ions are much more important than specific pH-conditions under practical paper-making conditions.

Kinetic schemes for the hydrolysis reactions are formulated and catalytic hydrolysis mechanisms are formulated. In the case of HCO3

-, a cyclical transition complex involving a proton transfer mechanism, activating the hydroxyl ion is suggested. For the metal ions, Lewis acid catalysis by coordination of metal ions on the acyl oxygen followed by activation of the ketone is suggested. In this case the nucleophilic attack is conducted by water instead of hydroxyl ion as Lewis catalysis of the hydrolysis reaction was found almost independent of pH from pH = 6 to pH = 10.

It was also shown that a sequestering agent, like EDTA, could inhibit the metal ion catalyzed hydrolysis.

Sammanfattning Betingelserna för hydrolys of katjonaktiv Alkyl Keten Dimer (AKD) dispersion har undersökts. Den undersökta AKD-dispersionen var C-14 märkt AKD, vilken vid hydrolys degraderas till den korresponderande ketonen, under avgivande av C14O2. Genom att mäta restaktiviteten i dispersionen, kan graden av hydrolys kvantifieras. Kinetiken för AKD-hydrolysen undersöktes vid olika temperaturer, pH samt kemisk miljö (olika elektrolyter). Som förväntat gick hydrolysen snabbare vid högre pH-värden.

Viktigare var dock upptäckten att närvaron av HCO3- och divalenta metaljoner

(Ca2+, Ba2+ and Mg2+) katalyserade AKD-hydrolysen. Katalysen, inducerad av dessa jonslag bedöms mycket viktigare än de specifika pH-betingelserna under praktiska paperstillverkningsbetingelser.

Ett kinetiskt schema för hydrolysreaktionen formulerades och två olika katalytiska hydrolysmekanismer föreslås. I fallet HCO3

-, föreslås att katalysen går över ett cykliskt complex, involverarande en protontransfer, som aktiverar hydroxyljonens nukleofila egenskaper. För metalljonerna föreslås en Lewissyrakatalys, som aktiverar koordinationen av metlljonerna till acylsyret, vilket i sin tur aktiverar de elektrofila




5 (24)

egenskaperna hos ketonen. I detta fall föreslås att det är vattenmolekylen istället för hydroxyljonen som utför den nukleofila attacken på ketonen eftersom den Lewis-syrakatalsyserade hydrolysen befanns i stort sett oberoende av pH från pH = 6 to pH = 10.

Det visades också att den metalljonkatalyserade hydrolysen kunde inhiberas genom komplexbildare som EDTA.




6 (24)

2 Introduction Alkyl ketene dimers (AKD) are commonly used for sizing of paper/board materials. It is well known that the sizing process occurs in three consecutive steps, namely retention, spreading and reaction. The sizing efficiency has been shown to be directly linked to the amount of reacted AKD (Lindström and Söderberg 1986) and size reten-tion will obviously be of critical importance. Recirculated AKD-size will become the subject to alkaline hydrolysis and therefore single pass retention is an important parameter for the improvement of the sizing process.

AKD-hydrolysis occurs when AKD reacts with water forming a β-keto acid, which spontaneously decarboxylates forming the corresponding ketone. AKD is, however, stable at room temperature at acidic pH-values, allowing storage at the time scale of months. There are very few quantitative studies of AKD-hydrolysis. Indeed, Jiang and Deng (2000) found that an increased pH increased hydrolysis. The hydrolysis is slow compared to hydrolysis of alkenylsuccinic acid (ASA), but is still of significance under practical papermaking conditions (Colasurado and Thorn 1992; Marton 1990, 1991), when process water with AKD is being recirculated in the mill.

Precipitated calcium carbonate (PCC) has also been shown to have a catalytic effect on AKD-hydrolysis (Jiang and Deng 2000). Residual amounts of alkali from PCC-manufacture are believed to be important for hydrolysis, but details remain unclear.

The hydrolysis product of AKD, the ketone, has a slight positive effect on sizing (Lindström and Söderström 1986a), provided there is some reacted AKD already present in the paper, but altogether it is the reacted AKD, which is the major con-tributor to the sizing.

The objective of this study was to investigate the role of different catalysts for AKD-hydrolysis.

This investigation is the fourth contribution in our second serie of papers aimed at examining the AKD-sizing process using C-14 labelled AKD. The methodology is not new and was used in this laboratory already many years ago in our first series of papers (Lindström and Söderberg 1986a, c-d; Lindström and O´Brian 1986b).

Previous papers in the latter series of papers have examined the role of differently bleached pulps on AKD-sizing (Johansson and Lindström 2004a), the role of a various retention aids and the state of agglomeration of AKD-particles on sizing (Johansson and Lindström 2004b) and the role of surface charge density and electrolyte concen-tration on AKD-sizing (Lindström and Glad-Nordmark 2006).




7 (24)

3 Materials manufacture

3.1 Preparation of C-14-labelled AKD-dispersion Standard C-14-labelled alkyl ketene dimer (AKD) (mp: 58-62º C) was made from a mixture of AKD wax (EKA Chemicals, Sweden) and C-14-labelled AKD wax (Amersham Pharmacia Biotech, Buckinghamshire, England). The dispersion was stabilized with a mixture of cationic starch (C-starch), HiCat 21370 (Roquette Freres, France), and lignosulfonic acid (LISA), Wanin S (Lignotech, Borregaard, Norway), according to a standard procedure given by EKA Chemicals and previously reported (Johansson and Lindström 2004). The fatty acid distribution was C18: 65 %, C16: 30 % and C14: 10 %.

3.2 Determination of AKD-hydrolysis The hydrolysis experiments were carried out in a three-necked glass vessel. All experiments were carried out using a 1.13 g/l AKD-dispersion. The glass vessel was heated to the appropriate temperature and the pH and electrolyte conc. was set. An aliquot of the dispersion was sampled at different times. The samples were then pH-adjusted to pH = 3, after which the liberated 14CO2 was flushed of the dispersion with N2. The residual activity in the AKD-dispersion was determined by soaking a filter paper with the AKD-dispersion, weighing the sample and oxidizing the filter paper in a Packard Sample Oxidizer Model 307 (Hewlett Packard, USA). The extent of hydrolysis can then be calculated from the residual radioactivity in the sample. A fully hydrolyzed sample loses 50% of it´s radioactivity.

The 14CO2 was absorbed in Carbosorb E (Packard BioScience Company, USA) and diluted with Permaflour E+ (Packard BioScience Company, USA) before the 14C-activity was measured in a Tri-Carb Liquid Scintillation Analyzer Model 2100TR.




8 (24)

4 Results and Discussion Figure 1 shows the effect of NaHCO3 on AKD-hydrolysis at 70 ºC at pH = 8. The hydrolysis is slow in the absence of NaHCO3 and increases with an increased NaHCO3 concentration. NaHCO3 is a well-known accelerator for the reaction between AKD and cellulose (Lindström and Söderberg 1986c) and it was also, suggested, based on the analysis of the kinetics, that the reaction cellulose-AKD reaction is catalyzed via a trimolecular reaction. In a similar fashion it is here suggested that the catalysis of hydrolysis also goes over the analogous trimolecular reaction path, shown in figure 2.

Figure 1 The effect of NaHCO3 on AKD-hydrolysis. Temperature = 70 ºC, pH = 8

Figure 2 A suggested mechanism for the catalysis of AKD-hydrolysis by NaHCO3

Figure 2 A suggested mechanism for the catalysis of AKD-hydrolysis by NaHCO3




9 (24)

Figure 3 Reaction rate plot (lnA = ln [a]/[a-x] = Kh t) for hydrolysis of AKD at different NaHCO3 concentrations. Temperature = 70 ºC, pH = 8

The kinetics of the hydrolysis reaction may be written as:

dx/dt = Kh [AKD] (1)

where x is the amount of hydrolyzed AKD and Kh is the rate of hydrolysis. Eq. (1) can be written as.

dx/dt = Kh [a-x] (2)

where a is the initial amount of AKD. Integration of eq. (2) yields.

lnA = ln [a]/[a-x] = Kh t (3) Hence, a plot of lnA vs time of hydrolysis (t) should yield a straight line and such a plot is shown in figure 3.

The reaction scheme in figure 2, suggests a trimolecular hydrolysis reaction and if this is the case, hydrolysis in the presence of NaHCO3, should be written as:

dx/dt = Kh [AKD] [NaHCO3] (4)

Similar experiments were performed at pH = 10, the results of which are displayed in figures 4 and 5. Analysis of the reaction hydrolysis constant, Kh also yielded straight lines at pH = 10 and the final results have been collected in figure 6. As shown in figure 6 the hydrolysis constant is reasonably linear with the NaHCO3 concen-tration, as suggested by eq. 4.




10 (24)

Figure 4 The effect of NaHCO3 on AKD-hydrolysis. Temperature = 70 ºC, pH = 10

Figure 5 Reaction rate plot (lnA = ln [a]/[a-x] = Kh t) for hydrolysis of AKD at different NaHCO3 concentrations. Temperature = 70 ºC, pH = 10




11 (24)

Figure 6 Rate of AKD hydrolysis (Kh) vs NaHCO3 concentration at different pH-values

The rate of hydrolysis should in a homogenous system be expected to be two orders of magnitude faster at pH = 10, than at pH = 8. In heterogeneous systems, the pH at the AKD-particle interface is different from in solution. For cationic particles, the pH is actually higher at the interface (attraction of OH¯ ions towards the cationic surface). The attraction is somewhat lower at pH = 10, because the particle is less cationic at pH = 10, because more acidic, negatively charged fatty acids are ionized at this pH. This is most likely the reason why the hydrolysis rate is not two orders of magnitude larger at pH = 10 compared with pH = 8.

The rate of hydrolysis is generally insignificant at acidic pH-values (figure 7), where bicarbonate ions are present.

The effect of temperature on the rate of hydrolysis is shown in figure 8. In this particular case the temperature effect was investigated in the presence of 0.1 M NaHCO3. It is obvious that at temperatures below 50 ºC, there is very little hydrolysis. This was also the reason why most hydrolysis experiments were conducted at 70 ºC, a temperature where appreciable hydrolysis occurs. This temperature is also in the upper range of temperatures occurring under practical papermaking conditions. As seen from figure 8 the rate of hydrolysis increases strongly with temperature.




12 (24)

Figure 7 The effect of pH on AKD-hydrolysis

Figure 8 The effect of temperature on the hydrolysis rate (Kh)




13 (24)

All these hydrolysis data could be understood from simple chemical considerations. It was, however, discovered during the course of these investigations that divalent metal ions also catalyzed the hydrolysis.

Hence, figure 9 shows the effect of CaCl2 on AKD-hydrolysis. The higher the CaCl2 concentration, the faster is the hydrolysis. In order to further analyse the data, the rate of hydrolysis (Kh) was calculated using eq. 3. The rate plots are given in figure 10. The lines are reasonable linear and the hydrolysis rate vs. CaCl2 concentration is given in figure 11. It appears as if the reaction rate increases strongly between 10-4 M CaCl2 and 10-3 M CaCl2. The plot gives, however, no information on any molecularity of the hydrolysis reaction in the presence of CaCl2.

Figure 9 The effect of CaCl2 on AKD-hydrolysis. Temperature = 70 ºC, pH = 8.




14 (24)

Figure 10 Reaction rate plot (lnA = ln [a]/[a-x] = Kh t) for hydrolysis of AKD at different CaCl2 concentrations. Temperature = 70 ºC, pH = 8.

Figure 11 Rate of AKD hydrolysis (Kh) vs CaCl2 concentration




15 (24)

This was a somewhat surprising discovery, both from a theoretical point of view and from a practical point of view, as this has apparently not been known before. To our knowledge there is no literature information on the effects of metal ions on AKD hydrolysis. It may be expected that ions compress the electrostatic double layer. Compression of a cationic double layer with an electrolyte would decrease the attrac-tion of hydroxyl groups into the double layer - a mechanism, if it was correct, would predict a decreased hydrolysis rate instead of the experimentally observed increase in the rate of hydrolysis.

The next issue was to investigate the specificity of the catalysis by divalent cations.

In one series of experiments, CaCl2 was compared with BaCl2 and MgCl2 with respect to their specificity in the catalysis reaction. The results are given in figure 12 and they show that all these metal ions catalyze the hydrolysis reaction. There are barely any significant differences between the different ions although the data show some peculiarities at intermediate hydrolysis times.

The next step was to investigate whether the cation or the anion was important for the hydrolysis. Firstly, using the magnesium salt the Cl-anion was shifted to the SO4

2- ion. As shown in figure 13, there is very little difference in the hydrolysis rate, when the chloride ion is shifted to the sulphate ion. Therefore, the nature of the anion seems irrelevant. Next, the electrolyte was shifted to Na2SO4. This salt was also used at a high concentration in order to indicate if screening effects on the AKD-emulsion particles had any effect. It is obvious that there is no catalytic effect of Na2SO4 on AKD-hydrolysis. Therefore, the catalytic effect seems to be related to the metal ions and have nothing to do with screening effects, which could affect the local pH at the emulsion interface.

Figure 12 The effects of CaCl2, BaCl2 and MgCl2 of on AKD-hydrolysis. Temperature = 70 ºC, pH = 8.




16 (24)

Figure 13 The effects of Na2SO4, MgCl2, MgSO4, AlCl3 and precipitated calcium carbonate (PCC) on AKD-hydro-lysis. Temperature = 70 ºC, pH = 8.

One series of experiments was also conducted to investigate the combined effects of pH and CaCl2. The results in figure 14 show that in the presence of CaCl2, the pH-effect is barely recognizable and the hydrolysis is basically catalysed by Ca-ions. The catalytic action of the metal ions may now be understood in term of Lewis acid catalysis by the metal ions. The Lewis acid coordinate with the free electrons on the acyl oxygen, withdrawing electrons from the ketone, thereby activating the ketone, so it can be subjected to nucleophilic attack from water. The reaction sequence is illustrated in figure 15.




17 (24)

Figure 14 Catalysis of hydrolysis of AKD in 10-3 M CaCl2 at different pH-values. Temperature = 70º C.

Figure 15 A suggested mechanism for the catalysis of AKD-hydrolysis by divalent metal ions (Lewis acid action)

It was also checked if it was possible to suppress the Lewis acid catalysis by complexation of the Ca2+-ions by a sequestering agent such as EDTA (Ethylene Diamine Tetra Acetic acid) as shown in figure 16. In this case a molar excess (2x) of EDTA was added and as shown in figure 16, hydrolysis was effectively terminated.

Two other experiments were also conducted. In one experiment AlCl3 was added in order to determine any effect of hydrolyzed metal ions on AKD-hydrolysis, but there is no effect of such additions on AKD-hydrolysis as shown in fig. 13. As both Ca2+-ions and HCO-

3 ions affect the hydrolysis it was also interesting to investigate the effect of calcium carbonate filler on hydrolysis. Other authors before have investi-gated the hydrolytic effect of calcium carbonate, but it was interesting to add this experiment to the context of this investigation. It is evident from figure 13 that there is a strong effect of PCC on hydrolysis.




18 (24)

Figure 16 The effect of a sequestering agent (EDTA) on catalysis of hydrolysis of AKD 10-3 M CaCl2. Temperature = 70º C, pH = 8.

In a last investigation, the issue was whether the metal ion content in real process waters has a significant impact on the hydrolysis rate, or if there are other factors, which are more important. Therefore the hydrolysis of AKD in process water from a CTMP-pulp (used as a middle layer in liquid packaging board) line from Skoghall Mill, StoraEnso (Karlstad, Sweden) was investigated.

This process water was not specifically analyzed for different metal ions, but instead the conductivity was measured and based on the conductivity, a calcium ion concen-tration was calculated (assuming that the major specie is Ca (HCO3)2), after which EDTA was added to complex twice the molar amount Ca calculated in this way.

The results are shown in figure 17, which shows that it is possible to complex the divalent metal ions and reduce hydrolysis in process waters. This also indicates that this may be an important practical tool to suppress hydrolysis under practical mill conditions.




19 (24)

Figure 17 The effect of EDTA-addition on hydrolysis of AKD in a process water from a CTMP-mill.




20 (24)

5 Conclusions In conclusion, this is the most extensive investigation, hitherto, on the hydrolysis of AKD and its catalysis by different ions. It was found that HCO-

3 ions and various metal ions (Ca2+, Ba2+ and Mg2+) had a strong catalytic effect on hydrolysis of AKD. The mechanisms of action of these ions have also been formulated. In the case of HCO-

3, a cyclical transition complex involving a proton transfer mechanism, activating the hydroxyl ion is suggested. For the metal ions, Lewis acid catalysis by coordination of metal ions on the acyl oxygen followed by activation of the ketone is suggested. In this case the major nucleophilic attack is conducted by water instead of hydroxyl ions as Lewis acid catalysis was only slightly dependent on pH.

It is also shown that the use of complexing agents can effectively inhibit the latter catalysis.




21 (24)

6 References Choon-Ki M and Dong-So S (1998) “A Quantitative Analysis Method for Studying AKD Hydrolysis” Journal of Korea Tappi, 30(3): 29-37

Colasurado A R and Thorn I (1992) “The Interactions of Alkyl Ketene Dimer with other Wet-End Additives” Tappi J., 75(9): 143–149

Isogai A (2000) “Stability of AKD-Cellulose Beta-Ketoester Bonds to Various Treatments” J. Pulp & Paper Sci., 26(9): 330-334

Jiang H and Deng Y (2000) “The Effects of Inorganic Salts and Precipitated Calcium Carbonate Filler on the Hydrolysis Kinetics of Alkylketene Dimer” J. Pulp & Paper Sci., 26(6): 208-213

Johansson J and Lindström T (2004a) “A Study on AKD-Size Retention, Reaction and Sizing Efficiency. Part 1: The Effects of Pulp Bleaching on AKD-sizing” Nordic Pulp Paper Res. J., 19(3): 330-335

Johansson J and Lindström T (2004b) “A Study on AKD-Size Retention, Reaction and Sizing Efficiency. Part 2: The Effects of Electrolytes, Retention Aids, Shear Forces and Mode of Addition on AKD-Sizing using Anionic and Cationic AKD-Dispersions” Nordic Pulp Paper Res. J., 19(3): 336-344

Lindström T and Glad-Nordmark G “A Study of AKD-Size Retention, Reaction and Sizing Efficiency. Part 3: The Effects of Fibre Charge Density and Electrolyte Conc. on Size Retention. Paper submitted to Nordic Pulp Paper Res. J.

Lindström T and Söderberg G (1986a) "On the Mechanism of Sizing with Alkylketene Dimers. Part 1. Studies on the Amount of Alkylketene Dimer Required for Sizing of Different Pulps” Nordic Pulp Paper Res. J., 1(1): 31

Lindström T and O'Brian H (1986) "On the Mechanism of Sizing with Alkyl Ketene Dimers. Part II. The Kinetics of Reaction between Alkyl Ketene Dimers and Cellulose”, Nordic Pulp Paper Res. J., 1(1): 26




22 (24)

Lindström T and Söderberg G (1986b) "On the Mechanism of Sizing with Alkyl Ketene Dimers. Part III. The Role of pH, Electrolytes, Retention aids, Extractives, Ca-lignosulfonates and Mode of Addition on Alkyl Ketene Dimer Retention” Nordic Pulp Paper Res. J., 1(2): 31

Lindström T and Söderberg G (1986c) "On the Mechanism of Sizing with Alkylketene Dimers. Part IV. The Effects of HCO3-ions and Polymeric Reaction Accelerators on the Rate of Reaction between Alkyl Ketene Dimers and Cellulose” Nordic Pulp Paper Res. J., 1(2): 39

Marton J R (1990) “Practical Aspects of Alkaline Sizing-On Kinetics of AKD reactions: Hydrolysis of AKD” Tappi, 11: 139-143

Marton J (1991) “Practical Aspects of Alkaline Sizing II. AKD in Mill Furnishes: Material Balances and Distribution” Proc. Papermakers Conf., April 1991, Seattle, WA, USA, p 405 Tappi Press, Atlanta, GA, USA


STFI-Packforsk AB Visit Drottning Kristinas väg 61 | Mail Box 5604, 114 86 Stockholm, Sweden

Tel +46 8676 70 00 | Fax +46 8411 55 18 | www.stfi-packforsk.se | [email protected] VAT no. SE556603110901


a study of akd-size retention, reaction and sizing … report 162.pdfaccording to...

Documents