Alkyl Ketene Dimer (AKD) Sizing Treatment and Charge Interactions in. Department of Forest Products Chemistry and Technology Istanbul University,

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1 Alkyl Ketene Dimer (AKD) Sizing Treatment and Charge Interactions in Recycled Paper AHSEN EZEL BİLDİK 1*, MARTIN A. HUBBE 2, and K. BAHATTIN GÜRBOY 3 1* Department of Forest Products Chemistry and Technology Istanbul University, Bahcekoy, Sarıyer, Istanbul 34473, Turkey 2 Department of Forest Biomaterials North Carolina State University, Raleigh, NC , ABD 3 Department of Forest Products Chemistry and Technology Istanbul University, Bahcekoy, Sariyer, Istanbul 34473, Turkey Corresponding author ahsenezel.bildik@istanbul.edu.tr SUMMARY Alkylketene dimer (AKD) sizing dispersions from two commercial sources, in addition to the corresponding laboratory-produced AKD dispersions, were investigated relative to their usage in a recycled office waste furnish. Two main sets of experiments were carried out. One set involved testing the pulp after AKD treatment, with the evaluation of dewatering rates, retention efficiency, and charge. The other set involved brightness and water resistance properties when AKD was added in making handsheets. There was generally a positive but decreasing incremental effect of the sizing treatments (dispersions or associated cationic polyelectrolytes) with increased levels of addition, on drainage rate and retention efficiency. AKD treatment resulted in increased brightness, which was attributed to increased retention of calcium carbonate and of fluorescent whitening agent in the paper. Less sizing agent was required in the recycled 1

2 furnish compared to the virgin fibre. Results were consistent with the charged character of the emulsified AKD formulations. KEYWORDS Alkylketene dimer sizing; Mixed office waste paper; Paperboard properties; Water resistance; Paper strength, Brightness. INTRODUCTION Paperboard production is an important part of the paper industry; it represents about 14% of Turkey s paper production (1), where the main raw material for this paperboard is recycled fibres. Use of recovered fibre in paperboard production (furnish) gives some advantages from environmental and cost effectiveness perspectives. A common utilization area of this paperboard is packaging. Therefore, the surface of the paperboard must be suitable for printing, thus requiring careful control of liquid penetration rates. Recycling processes lead to decreased strength properties of fibres, which can make it more challenging to achieve the high quality requirements of present-day packaging products. In order to overcome this deficiency of properties of recycled fibres, papermakers use a variety of chemical additives. In addition, sizing chemicals provide water repellent character to the paper in order to impart paper and paper products with a certain degree of water resistance against the liquid penetration of substances such as water, aqueous ink, coffee, and acidic juices (2). Control of the penetration of liquid can be achieved through a combination of treatments to both the paper web as a whole (internal sizing) and to the paper surface (surface sizing). Because internal sizing requires addition of the sizing agent to the fibre slurry, 2

3 interactions between the sizing agent and other components of the system can be critical to success (3). For instance, retention aids, impurities, and various other additives can influence the retention of the sizing agents in the paper and also the extent of aggregation of the particles and fibres. Electrical charge is a key factor affecting the retention and performance of chemical additives to the papermaking process (4). For instance, if the fibre surfaces and the surfaces of a dispersed additive have opposite signs of charge, then efficient retention can be expected. The zeta potential of fibres, which represents the electrical potential at a slip plane adjacent to the surface, was evaluated in this work by fibre-pad streaming potential measurements (5). Most previous studies involving AKD sizing have dealt with treatments of virgin furnish, and only a few have considered the charged character of the sizing formulation (6). The present work systematically considers the effects of several interactions that are particularly encountered when using recycled office paper furnish in the production of paperboard. Outcomes including retention, dewatering rates, and the development of hydrophobicity are considered in the light of interactions among electrically charged ingredients in the suspension. The present findings, which can provide guidance for the production of paperboard from recycled office paper, also reveal some wider implications for papermaking. EXPERIMENTAL SYSTEM Materials The fibre source used in the study was Recycled (100%) Envirocopy office paper with 9% ash content. No additional fillers were used. 3

4 The pulp was produced by disintegrating the 100% recycled office paper according to TAPPI Method T205 (using a TMI disintegrator, 400 Bayview Ave. Amityville NY 11701). The pulp was adjusted to 0.50% consistency and stored at 25 ºC in a 8000 ml container. Sodium sulphate (Fisher Scientific LOT number: ) was added to reach a conductivity of 1006 µs/cm at 23 ºC, so as to maintain a salt concentration that is within a typical range used in commercial manufacturing of paper. As sizing agents, two different forms of AKD (solid and emulsified) were each obtained from two sources (AKD-1 from Solenis (A364), and AKD-2 from Disproquin. S.A.S. Lot:1413ST331343). Emulsified AKD was used as is in the laboratory preparation of paper sheets (in handsheet preparation). In order to compare the commercial and laboratory emulsified AKD, the AKD wax (solid) samples were used to prepare dispersions under laboratory conditions. The AKD wax was emulsified to form a dispersion in a cationic starch solution. AKD-1L and AKD-2L represent the emulsified AKD samples prepared in the laboratory. On the other hand, AKD-1C and AKD-2C represent AKD dispersions prepared by commercially. AKD-1C includes 20%, and AKD-2C includes 15% content of active AKD. Cationic starch, as provided by Disproquin S.A.S., was used as a stabilizer in the preparation of AKD dispersions (AKD-1L and AKD-2L). The cationic acrylamide retention aid used in the work was Percol 175, as obtained from BASF Corp. (Material number was and LOT number was ). Certain sets of the tests were intentionally carried out without addition of cationic PAM in order to distinguish the differing effects of AKD formulation components on fine particle retention. 4

5 EQUIPMENT Handsheets were prepared with a British sheet mould according to a TAPPI method (T 205 sp-02). The white sheets were prepared with a basis weight of 60 gsm, similar to the grammage of the top ply of commercial white top cardboard. Therefore, it will be possible to compare the results for the all the paper having basis weight in the range gsm. The default drying conditions were 105 ºC heating on an Adirondack Dryer, with the final moisture content reduced to about 1.50%. Optionally, some tests were carried out in which the drying conditions were varied, e.g. room temperature 25 ºC, rotary dryer at 75 ºC, 120 ºC, 150 ºC, and supplemented with oven drying 105 C. Test Conditions In preparation for testing, the pulp was stirred with an impeller speed of 500 rpm (using a Yamato LR400D stirrer). First the AKD dispersion was added to the pulp and then the cationic acrylamide retention aid (Cationic PAM) was added. The mixing time was 30 seconds. Different amounts of Cationic PAM were added to the pulp and the effect on the freeness was evaluated. Preparation of AKD dispersions Cationic starch dispersion was prepared with 98 g water and 1 g cationic starch. Before making the dispersion, the starch was cooked at 95 ºC using a hot plate. A commercial type blender vessel was placed in a convection oven, where it stayed for 2 h at 95 ºC. After the cooking of the starch, it was immediately put it in a pre-heated stainless steel blender. Then the 1 g of AKD was added into the blender (Fig. 1). After the mixture had 5

6 been blended for 60 s, it was diluted with ice water to a final concentration of 0.50% (w/v). Figure 1. Preparation of AKD Dispersion. Drainage Results with Modified Schopper-Riegler Method In the present work the refining level of the fibres was not varied, so the only effects to be considered involved effects of chemical additives. For such evaluations it has been shown that a modified Schopper Reigler type drainage test can be advantageous (8). As noted by Sampson and Kropholler (9-10), who developed the test at the University of Manchester, it is important to utilize a test procedure that does not depend on the resistance of flow of filtrate through a narrow capillary, as is the case for conventional freeness tests. Rather, one monitors the mass of total filtrate as a function of time. The modified Schopper-Reigler test device was thus used to determine how different amounts of polyacrylamide-based polymer (Cationic PAM), AKD and Cationic Starch 6

7 affect the drainage rate. The present work employed a portable version of the modified Schopper Reigler drainage tester (Buckman Laboratories). Retention Efficiency (Britt Jar) The efficiency of retention of fine particles at different levels of additives was evaluated with a dynamic drainage/retention jar, as described by Britt and Unbehend (11-12). This test has been widely used for analysing wet end chemistry, with emphasis on retention aid treatments for the paper machine. Moreover, recent studies show that there is a valuable correlation obtained between turbidity and the concentration of fines (13-14). Turbidity tests were used in this work to indicate relative changes in retention. The consistency of pulp used in these tests was similar to that present in the headbox of a paper machine (0.50%). The jar was filled with 700 ml of pulp slurry. After starting the stirrer (500 rpm), AKD was added at the selected level, and then cationic PAM as a 0.10% solids solution was added at the 0.05% level based on solids. Stirring was continued for 30 s before sampling. Then 20 ml of filtrate was withdrawn through the eyedropper of the device and returned to the jar. This was repeated with another 20 ml of filtrate, which was taken for analysis. An assumption was made that at suitable low concentrations there will be a linear relationship between filtrate solids and turbidity. The latter quantity was evaluated with a (DRT-15CE turbidimeter from HF Scientific). Zeta Potential Zeta potential measures the electrical interaction between particles. It is important to understand how the variety of additives and fines effect these interactions, and in particular how flocculation can be optimised. Here the zeta potential measurements were aimed at observing the chemical interactions between AKD and components of the 7

8 recycled pulp, in particular the fine particles in the filtrate from the hand sheet former and the calcium carbonate filler, and also the cationic polymers present.. Streaming potential tests were used as a means of estimating the zeta potential at the surface of the fibres (5). Because streaming potential data are known to depend strongly on the electrical conductivity of the supporting medium, a solution concentration of 1006 µs/cm was used for all experiments. The Helmholtz-Smoluchwoski equation was used for the calculation of zeta potential from the streaming potential values. Size Tests The Hercules size test, as employed in this work, is widely used for evaluating the water resistance of paper grades, provides a measure of the amount of time needed for acidified water to permeate through a sheet. The test senses the time needed for the reflectance of the paper specimen to decrease to 80% of its initial value after being contacted from its opposite side with a coloured aqueous solution that contains 1% formic acid (TAPPI Method T530). ISO Brightness Optical properties, primarily brightness, is important for the paper to be printed. The recycling process itself has a deleterious effect on the optical properties. Drying conditions also affect the optical properties (7). Sizing treatments, as required by some applications, also have the potential to affect the paper s appearance. In this work the optical properties such as brightness, whiteness and L, a*, b* color space values were evaluated using Technidyne Color Touch 2 spectrophotometer with D65 illuminant. 8

9 Filtrate Mass [g] RESULTS AND DISCUSSION Evaluations of Drainage Rates As can be seen from Figure 2, the rate of dewatering increased with increasing addition levels of the cationic retention aid. It is worth noting that the addition levels were selected to be in a geometric progression. Even a relatively low dosage of 0.01% on a solids basis was enough to achieve a consistent increase in the amount of filtrate obtained at each of the times of observation. Although the highest dewatering rate corresponded to the highest addition level of the retention aid (0.50%), most of the benefit was already achieved at one-tenth of that addition level (0.05%) Cat. PAM Cat. Starch Drainage Time [s] Blank 0.01% 0.02% 0.05% 0.10% 0,20% 0,50% Fig. 2. Cationic PAM and cationic starch addition versus filtrate mass during freeness testing. Two types of AKD (emulsified commercially and in the laboratory) from two different manufacturing sources (AKD-1 and AKD-2) were compared. These included the commercial type (C), which was supplied in dispersion form, and a laboratory-prepared 9

10 Filtrate Mass [g] Filtrate Mass [g] (L) dispersions of AKD. Drainage curves for AKD-1 for both the C and L dispersions are shown in Figure 3.. Corresponding results for AKD-2 are shown in Figure (AKD-1C) (AKD-1L) Drainage Time [s] Blank 0.01% 0.02% 0.05% 0.10% 0.20% 0.50% Fig. 3. AKD-1 commercially dispersed (AKD-1C) and laboratory dispersed (AKD-1L) addition versus filtrate mass during freeness testing 1000 (AKD-2C) (AKD-2L) Drainage Time [s] Blank 0.01% 0.02% 0.05% 0.10% 0.20% 0.50% Fig. 4. AKD-2 commercially dispersed (AKD-2C) and laboratory dispersed (AKD-2L) addition versus filtrate mass during freeness testing When considering the results from the modified Schopper-Riegler tests in Figures 3 and 4, it is clear that addition of any of the sizing dispersions yielded increased rates of 10

11 dewatering. Moreover, there was a consistent difference between the results for the commercial dispersions of AKD and the laboratory-prepared dispersions of all types (with AKD-1, AKD-2). The latter samples all illustrated a progressive increase in drainage rate with increasing dosage of dispersion to the fibre suspension. By contrast, addition of the commercial dispersions of AKD sizing agent yielded a relatively constant and high rate of dewatering regardless of treatment dosage, within the ranges considered. It is proposed that the different behaviour may be due to a different composition of commercial dispersions in comparison to the simple cationic-starch-based dispersions represented in Figure 2. Though the composition of the commercial dispersions are proprietary, it is known that high-charge cationic additives are frequently employed. A high-charge cationic polymer ingredient can be expected to neutralize the negatively charged colloidal material in a pulp suspension at a lower addition level, thus bringing about a strong increase in the rate of dewatering. Retention Efficiency (Britt Jar) Different levels of cationic PAM retention aid (Cat. PAM) were employed. Effects of treatment at the following addition levels were evaluated (based on furnish solids): zero (Blank), 0.01%, 0.02%, 0.05%, 0.10%, 0.20%, 0.50%, 1.00%, 2.00% and 5.00%. Results are shown in Figure 5. 11

12 Turbidity [NTU] Addition Amount [%] AKD-1L AKD-2L Cat. Starch Fig. 5. Laboratory dispersed AKD-1and AKD-2 addition versus turbidity Although Figure 5 appears to show a nearly linear decline in filtrate turbidity with increasing addition level of each agent or each dispersion, one must bear in mind that the horizontal axis is based on a geometric progression, i.e. an approximately logarithmic scale. Thus, these figures all indicate a system in which the relative effects on turbidity decreased with increasing level of addition of each agent or dispersion. However, within the range of conditions tested, there was no indication of a reversal of effects. Zeta Potential As shown in Figure 6, the largest changes in calculated zeta potential were generally associated with the first added amount of cationic substance (sizing dispersion or cationic 12

13 Zeta Potential [mv] starch). An exception was in the case of AKD-2 samples, for which the largest incremental changes were associated with higher levels of treatment Addition Amount [%] AKD-1C AKD-1L AKD-2C AKD-2L Cat. Starch Fig. 6. Zeta potential at increasing levels of commercial (C) and laboratory dispersed (L) AKD-1, AKD-2 and cationic starch. Considering retention tests, drainage tests, and zeta potential tests there did not appear to be an increasing effect related to dispersions or cationic polyelectrolytes. Initially the fibre surfaces were affected by strong adsorptive interaction, followed by a decreasing effect. It is hypothesized that the first incremental amounts adsorbed onto favourable sites at the fibre surface, and that once such sites were filled, the subsequent adsorption was not as favourable. Interestingly, none of the results showed evidence of reversal of charge of the surfaces or of reversal in drainage or retention effects with increasing treatment level. Thus, rather than saturating the surface, the added materials appeared to mainly interact just with favourable sites. 13

14 HST [s] HST [s] Results of Sizing Tests Results from sizing tests are shown in Figures 7 and 8. In each case, results are compared for four conditions of drying: room temperature 25 ºC, on a rotary dryer at 75 ºC, in an oven for 5 minutes at 105 ºC and on a rotary dryer at 150 ºC (AKD-1L) (AKD-1C) Addition Amount [%] 25 C 75 C 105 C 150 C Fig. 7. Hercules sizing test versus addition rate of commercially dispersed AKD- 1 (AKD-1C) and laboratory dispersed AKD-1 (AKD-1L) at different drying conditions (AKD-2L) (AKD-2C) AKD Addition Amount [%] 25 C 75 C 105 C 150 C Fig. 8. Hercules sizing test versus addition rate of commercially dispersed AKD- 2 (AKD-2C) and laboratory dispersed AKD-2 (AKD-2L) at different drying conditions. 14

15 Results shown in Figures 7 and 8 are remarkable insofar as they reveal substantial resistance to water permeation even at the lowest level of treatment, 0.02%. Based on other work (15-20), an addition level of 0.02% is often regarded as being below the threshold level for achieving significant increases in hydrophobic sizing effects. A likely explanation is that the present study was carried out with recovered fibres that previously had been hydrophobically sized during at least one prior cycle of papermaking. Thus, as shown elsewhere (21), one can expect that a lesser amount of newly added sizing agent would be needed to reach the threshold level when using the recycled fibres. The other remarkable aspect about the data in Figures 7 and 8 is that substantial sizing was achieved even when the paper was dried at room temperature. In other words, hydrophobic character was achieved even in the absence of heat-curing. This effect is tentatively attributed to the hydrophobic character of the AKD and the ability of the other additives to hold the AKD (or its decomposition products) at the paper surface. Somewhat higher levels of resistance to wetting were achieved in the heated samples, with the best results obtained with oven curing at 105 ºC, which might be regarded as representing commercial papermaking conditions. ISO Brightness results As shown in Figure 9, the sizing treatments had no significant effect on brightness at lower addition levels but highest levels of treatment consistently showed slightly higher brightness. The sizing agent treatments considered in this work are not intended to change the appearance properties of the resulting paper. However, it would be a concern if the sizing treatments decreased paper brightness. This might happen, for instance, if the cationic treatments (due to cationic starch or cationic retention aid) increased the retention 15

16 ISO Brightness of chromophoric materials present in the process water. Alternatively, this might happen if one or more of the components in the sizing treatment absorbed a significant amount of light. 87 PART-A (AKD-2L) 87 PART B (AKD-2C) Addition Amount (%) 25 C 75 C 105 C 150 C Fig. 9. ISO Brightness levels versus addition level for AKD-2L and AKD- It is reasonable to expect that increasing treatment levels with a cationic AKD dispersion would increase the retention of calcium carbonate, as well as of fluorescent whitening agent present in the recovered paper suspension. Although the present results appear encouraging in this regard, with respect to treatment of recovered office paper, it should be kept in mind that the furnish employed in this work had not been printed in its last cycle of use. If significant levels of toner ink had been present in the suspension, then an increased retention efficiency would have been expected to reduce the brightness of the resulting paper. CONCLUSIONS Results related to drainage, fine-particle retention, brightness, and hydrophobic sizing were obtained for both commercially dispersed and laboratory dispersed sizing agents, as 16

17 well as with cationic starch. This is the first time that a systematic study of AKD sizing has been carried out with recycled office paper as the furnish, with attention paid to the charged character of the system. Effects were generally consistent with the positive charge of the AKD dispersions, which will affect interactions with other components of a papermaking furnish. In addition to increasing the water-resistant properties of paper, it was shown that treatment of recycled office paper furnish with alkylketene dimer (AKD) dispersions can affect the papermaking process and other attributes of paper quality. These effects were evaluated with AKD dispersions from two commercial sources, and also for the corresponding laboratory dispersions, using with the original AKD wax from each of the same commercial sources. Though there was some variability among the results corresponding to the four dispersion types, the general trends indicated increased retention efficiency and drainage rates with increasing addition of AKD dispersions. Results were found to be consistent with a positive charged nature of the AKD dispersions, such that AKD dispersion addition tended to make the solid surfaces in the fibre suspensions less negative. ACKNOWLEDGEMENTS Financial support for this research provided from TUBITAK (2214-A, 110O B ). REFERENCES (1) Turkish Pulp and Paper Industry Foundation, Annual Report. İstanbul. 17

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19 (20) Isogai, A. and Morimoto, S. Sizing Performance and Hydrolysis Resistance of Alkyl Oleate Succinic Anhydrides, Tappi J. 3(7): 8-12 (2004). (21) Wang, T., Simonsen, J. and Biermann, C. J. A New Sizing Agent: Styrenemaleic Anhydride Copolymer with Alum or Iron Mordants, Tappi J. 80(1): (1997). 19