ahsen ezel bİldİk1*, martin a. hubbe - projects.ncsu.edu · 1 author version. citation: appita...

1

Author version. Citation: APPITA Journal 69(4), 331-338

Alkyl Ketene Dimer (AKD) Sizing Treatment and Charge Interactions in

Recycled Paper

AHSEN EZEL BİLDİK1*, MARTIN A. HUBBE2, and K. BAHATTIN GÜRBOY3

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 27695-8005, ABD

3 Department of Forest Products Chemistry and Technology

Istanbul University, Bahcekoy, Sariyer, Istanbul 34473, Turkey

Corresponding author e-mail: [email protected]

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

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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,

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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.

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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: 108250) 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 555479922 and LOT number was 0011598075).

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.

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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 50-

60 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 105C.

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

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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

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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

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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.

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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%).

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

0

200

400

600

800

1000

0 5 10 20 30 40 50 60 0 5 10 20 30 40 50 60

Filt

rate

Mas

s [g

]

Drainage Time [s]

Cat. PAM Cat. Starch

Blank 0.01% 0.02% 0.05% 0.10% 0,20% 0,50%

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(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 4.

Fig. 3. AKD-1 commercially dispersed (AKD-1C) and laboratory dispersed

(AKD-1L) addition versus filtrate mass during freeness testing

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

0

200

400

600

800

1000

0 5 10 20 30 40 50 60 0 5 10 20 30 40 50 60

Filt

rate

Mas

s [g

]

Drainage Time [s]

(AKD-1C) (AKD-1L)

Blank 0.01% 0.02% 0.05% 0.10% 0.20% 0.50%

0

200

400

600

800

1000

0 5 10 20 30 40 50 60 0 5 10 20 30 40 50 60

Filt

rate

Mas

s [g

]

Drainage Time [s]

(AKD-2C) (AKD-2L)

Blank 0.01% 0.02% 0.05% 0.10% 0.20% 0.50%

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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.

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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

0

10

20

30

40

50

60

0.00 0.01 0.02 0.05 0.10 0.20 0.50 1.00 2.00 5.00

Turb

idit

y [N

TU]

Addition Amount [%]

AKD-1L AKD-2L Cat. Starch

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starch). An exception was in the case of AKD-2 samples, for which the largest

incremental changes were associated with higher levels of treatment.

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.

-22

-18

-14

-10

-6

-2

Zeta

Po

ten

tial

[m

V]

Addition Amount [%]

AKD-1C AKD-1L AKD-2C AKD-2L Cat. Starch

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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.

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.

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.

0

100

200

300

400

500

0 0.01 0.02 0.05 0.10 0.20 0.50 0 0.01 0.02 0.05 0.10 0.20 0.50

HST

[s]

Addition Amount [%]

(AKD-1L) (AKD-1C)

25 ˚C 75 ˚C 105 ˚C 150 ˚C

0

100

200

300

400

500

600

0 0.01 0.02 0.05 0.10 0.20 0.50 0 0.01 0.02 0.05 0.10 0.20 0.50

HST

[s]

AKD Addition Amount [%]

(AKD-2L) (AKD-2C)

25 ˚C 75 ˚C 105 ˚C 150 ˚C

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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

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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.

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

82

83

84

85

86

87

0.00 0.10 0.20 0.30 0.40 0.50

ISO

Bri

ghtn

ess

Addition Amount (%)

PART-A (AKD-2L)

25 ˚C 75 ˚C 105 ˚C 150 ˚C

82

83

84

85

86

87

0.00 0.10 0.20 0.30 0.40 0.50

PART B (AKD-2C)

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, 110O558

1059B141300861).

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ahsen ezel bİldİk1*, martin a. hubbe - projects.ncsu.edu · 1 author version. citation: appita...

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