Alkyl Ketene Dimer and Precipitated Calcium Carbonate Interactions in Wetend Papermaking

8/12/2019 Alkyl Ketene Dimer and Precipitated Calcium Carbonate Interactions in Wetend Papermaking

1/112

Alkyl ketene dimer and precipitated calcium carbonateinteractions n wet-end papermaking

byAgatha Poraj-Kozminski

Department of Chemical EngineeringMcGill University MontrealMarch 2006

A thesis submitted to McGiII University n partial fulfillment of therequirements of the degree of Master of Engineering

Agatha Poraj-Kozminski2 6


2/112

1 1 Library andArchives Canada Bibliothque etArchives CanadaPublished HeritageBranch Direction duPatrimoine de l dition395 Wellington StreetOttawa ON K1A ONCanada

395, rue WellingtonOttawa ON K1A ONCanada

NOTICE:The author has granted a nonexclusive license allowing Libraryand Archives Canada to reproduce,publish, archive, preserve, conserve,communicate to the public bytelecommunication or on the Internet,loan, distribute and sell thes esworldwide, for commercial or noncommercial purposes, in microform,paper, electronic and/or any otherformats.The author retains copyrightownership and moral rights inthis thesis. Neither the thesisnor substantial extracts from itmay be printed or otherwisereproduced without the author spermission.

ln compliance with the CanadianPrivacy Act some supportingforms may have been removedfrom this thesis.While these forms may be includedin the document page count,their removal does not representany loss o content from thethesis.

Canada

AVIS:

Your file Votre rfrenceISBN: 978 0 494 25006 8Our file Notre rfrenceISBN: 978 0 494 25006 8

L auteur a accord une licence non exclusivepermettant la Bibliothque et ArchivesCanada de reproduire, publier, archiver,sauvegarder, conserver, transmettre au publicpar tlcommunication ou par l Internet, prter,distribuer et vendre des thses partout dansle monde, des fins commerciales ou autres,sur support microforme, papier, lectroniqueet/ou autres formats.

L auteur conserve la proprit du droit d auteuret des droits moraux qui protge cette thse.Ni la thse ni des extraits substantiels decelle-ci ne doivent tre imprims ou autrementreproduits sans son autorisation.

Conformment la loi canadiennesur la protection de la vie prive,quelques formulaires secondairesont t enlevs de cette thse.Bien que ces formulairesaient inclus dans la pagination,il n y aura aucun contenu manquant.


3/112

BSTR CT

This thesis investigates the interactions between alkyl ketene dimer AKD)and precipitated calcium carbonate PCC). Although the mechanisms behind AKDsizing and reactions with cellulose have been studied in-depth, methods describingAKD retenti on are poorly understood. The aim o this research was to determine theconditions and time-scale under which AKD and PCC heteroflocculate, and todetermine the influence o PCC on non-retained AKD. We also wanted to understandthe mechanisms behind AKD interactions with cP AM, and perform experiments onthe twin-wire sheet former. We outline a procedure for creating an AKD emulsioncoated by cationic starch and free o extraneous substances. We find that AKD andPCC each homoflocculate, but no heteroflocculation occurs between the twochemicals. This suggests that PCC and AKD do not directly interact. Instead, starchremoval from the AKD partic1e surface aids the homoflocculation o PCC viapolymer bridging). This indicates that the alkaline environment is the most significantfactor contributing to AKD hydrolysis. More importantly, we find that AKD and PCCare not likely to interact in the whitewater cycle. This refutes the general idea thatPCC lowers AKD retention. Adsorption kinetic experiments revealed that althoughcationic AKD and cP AM do not flocculate, cP AM does increase the retention oAKD by assisting its deposition onto fibers. Asymmetrical polymer bridging explainsboth the increase in AKD retention and the behavior o AKD flocculation kineticswith cPAM Although the initial kinetics are nearly independent o cP AM dosage,excess cP AM delays the achievement o maximum possible AKD retention due tostarch and cP AM re-conformation. Lastly, it was found that the addition o cP AMresults in an increase in the bond strength between AKD and fibers, therebyeliminating AKD detachment. Studies on the twin-wire sheet former found thatcP AM increases the first-pass retenti on o AKD three-fold. The twin-wire former wasproven to be a useful tool for studying AKD retention.


4/112

R SUMLe sujet de cette thse porte sur l tude des interactions entre le dimre de

ctne alkyl (AKD) et le carbonate de calcium prcipit (PCC). Bien que lesmcanismes de l encollage l AKD et de sa raction avec la cellulose aient ttudis en profondeur, ceux qui gouvernent la rtention de l AKD sont encore maldfinis. Nos recherches visaient dterminer les conditions et l chelle de temps souslesquelles l AKD et le PCC htro-floculent et dterminer l effet du PCC surl AKD non retenu. Nous voulions aussi comprendre les mcanismes de l interactionde l AKD avec le polyacrylamide cationique (cPAM) et raliser des expriences surla machine p apier double toile. Nous dcrivons un procd pour crer unemulsion d AKD englob par de l amidon cationique sans aucune autre substance.Nous avons observ que l AKD et le PCC homo-floculent, mais qu il n y a pasd htro-floculation entre les deux produits. Ceci indique que le PCC et l AKDn interagissent pas directement. Ce qui se produit plutt est que l amidon dtach dela surface des particules d AKD contribue l homo-floculation du PCC (par le biaisde ponts polymriques). Ceci indique qu un environnement alcalin est le facteurdominant qui contribue l hydrolyse de l AKD. De plus grande importance est notredcouverte que l AKD et le PCC sont peu enclin interagir dans le cycle de l eaublanche. Ceci rfute la thorie que le PCC rduit la rtention de l AKD. Desexpriences de cintique d adsorption ont rvl que mme si l AKD cationique et lecP AM ne floculent pas, le cPAM augmente la rtention de l AKD en contribuant sadposition sur les fibres. La formation de ponts de polymres asymtriques explique la fois la hausse de la rtention de l AKD et le comportement de la cintique defloculation de l AKD avec le cPAM. Mme si la cintique initiale est indpendantedu dosage de cPAM, un excs de cPAM retarde l atteinte du niveau maximal dertention cause de la re-conformation de l amidon et du cPAM. Enfin, nous avonsdcouvert que l ajout de cPAM augmente la force des liens entre l AKD et les fibres,ce qui empche le dtachement de l AKD. Nos expriences sur la machine papier double toile dmontrent que le cP AM multiplie par trois le niveau de rtention depremire passe de l AKD. La machine papier double toile s avre tre un outilutile pour l tude de la rtention de l AKD.


5/112

cknowledgements1 would like to thank Dr. Theo van de Ven and Dr. Reghan Hill for their continuedguidance and encouragement and for their dedication to the project. They have mademe a better researcher and 1 am etemally grateful to them for their support.

My sincere thanks to my colleagues at the Pulp and Paper Center and particularly tothe members ofmy research group: Lojza Marcius Jimmy Prasad and Meng.

Many thanks to Louis Godbout Dr. Alince Dr. Petlicki Lou Cusmich HelenCampbell and d Siliauskas for their invaluable contributions to this project. Aspecial thanks to Christopher Hammock creator of the laboratory twin-wire former.

My sincere gratitude to B. Sithole J. Pimentel and A. Gagne at Paprican PointeClaire for their help with AKD analysis.

And finally thank you to my parents and friends and to my husband Christopher forall your love and support and without whom none of this would have been possible.

IV


6/112

T BLE O CONTENTS

bstractResumAcknowledgementsContentsList of FiguresList of Tables

Chapter 1: Introduction1 1 Background1.2 Objectives of the Thesis1.3 Literature Review

1.3.1 Major Chemical Components1.3.1.1 Alkyl ketene dimer AKD)1.3.1.2 Precipitated Calcium Carbonate PCC)1.3 .1.3 Retention Aids1.3.1.4 Fibers

1.3.2 Retention Mechanisms1.4 References

Chapter 2: KD and PCC Interactions2.1 Abstract2.2 Introduction

v

ii

ivvixXlI

2

77810

12

13

14

17

18


7/112

2 3 Emulsification o AKD Stabilized with Cationic Starch 192 3 1 Materials and Methods 20

2 4 Emulsion Analysis 22 4 1 Particle Size Analysis 22 4 2 Electrophoretic Mobility Analysis 232 4 3 Effect o Starch Concentration and Ratio 252 4 4 Comparison to Commercial Emulsion 27

2 5 Photometrie Dispersion Ana1yzer Experiments 272 5 1 Materials and Methods 282 5 2 PDA Parameters 302 5 3 Results and Discussion 32

2 5 3 1 PCC Homoflocculation 322 5 3 2 PCC and AKD F10cculation 322 5 3 3 PCC and Starch Flocculation 342 5 3 4 Effects o Starch filtration 36

2 6 Scanning Electron Microscopy Analysis 392 7 Conclusions 422 8 Acknowledgements 432 9 References 44

VI


8/112

Chapter : AKD Interactions with cP M and Fibers3 1 Abstract3 2 Introduction3 3 Materials and Methods3 4 Materials

3 4 1 Methods: Photometric Dispersion Analyzer3 4 2 Methods: Adsorption Kinetics

3 5 Results3 5 1 AKD Flocculation3 5 2 Cationic AKD Deposition on Fibers3 5 3 Anionic AKD Deposition on Fibers

3 6 Conclusions3 7 Acknowledgements3 8 References

Chapter 4: Interactions o AKD and Other Papermaking Additives on aLaboratory Twin wire Sheet Former Machine4 1 Abstract4 2 Introduction4 3 Machine Description4 4 Materials and Methods4 5 Analysis ofResults4 6 Outline ofExperiments

vu

4748505051

525353546626264

666767727475


9/112

4.6.1 Reproducibility Experiments 754.6.2 PCC Retention with Cationic Retention Aid 774.6.3 First Pass Retention vs Second Pass Retention with AKD 784.6.4 AKD Retention at Various Additions 794.6.5 AKD Retention with cP AM 79

4.7 Conclusions 804.8 Acknowledgements 814.9 References 82

Chapter : Conclusions5 1 Overview 845.2 Recommendations for Further Work 87

Appendix A. Principles o Flocculation 89Appendix B. Starch Clustering and Effects o Shear and Sonication 93Appendix C. Twin-Wire Sheet Former Machine TWF) Instructions 96

V111


10/112

LIST O FIGUR S

CH PTERIFigure 1.1: Typical Approach System o a Paper Machine 3Figure 1.2: AKD p-esterification with cellulose 8Figure 1.3: Hydrolysis o AKD to palmitone 9

CH PTERIIFigure 2.1: Partic1e-size distribution o AKD emulsion 22Figure 2.2: SEM image o AKD emulsion at 1O,000X magnification 23Figure 2.3: Theoretical and experimental values o AKD partic1e diameter 25Figure 2.4: PDA arrangement with circulation and pump 29Figure 2.5: RATIO o dispersed versus aggregated partic1es 30Figure 2.6 a: Effect o varying flowrate on the flocformation in PDA for a stirring rate o 150 rpm 3Figure 2.6 b: Effect ofvarying stirring speed in beaker on the flocformation in PDA for a flowrate o 150 mL/min. 32Figure 2.7: PCC flocculation with AKD emulsion 34Figure 2.8: PCC flocculation with sonicated starch (low starch additions) 35Figure 2.9: PCC flocculation with sonicated starch (high starch additions) 35Figure 2.10: Schematics o energy o interaction between two PCC partic1es:

a) partially coated by s tarch- van der Waals and electrostatic attractionb) fully coated- van der Waals attractions and steric repu1sions 36Figure 2.11: Successive filtration schematic o AKD drop lets and starch stabilizer 37Figure 2.12: PCC flocculation with AKD2 (AKD filtered twice) 39Figure 2.13: SEM image o PCC partic1es 39

IX


11/112

Figure 2.14: SEM image o AKD emulsion 40Figure 2.15 a,b,c): PCC and AKD at a 1:1 Ratio 41Figure 2.16: SEM image o AKD emulsion after filtering away free starch;

AKD drop lets aggregate 43

CH PTERIIIFigure 3.1: Photograph o vials with various quantities ofcPAM;Amounts are given as mg cPAM/g anionic AKD. 53Figure 3.2: AKD deposition onto fibers, at 10 mg AKD/g fiber addition,

and varying cPAM addition. Inset: [mg cPAMlg fibers] 54Figure 3.3: AKD deposition to fiber surface 55Figure 3.4: AKD deposition on fibers

a low cPAM dosages; b) high cPAM dosages 56Figure 3.5: Asymmetric Polymer Bridging Mechanism 57Figure 3.6: Increased Bond Strength by Starch Re-conformation

a Initial adsorption o starch to cP AMb) Spreading o starch 58

Figure 3.7: Adsorption Kinetics o Anionic AKD emulsion to fibers,in the presence o cP AM.. 61

CH PTERIVFigure 4.1: Laboratory Twin-Wire Sheet Former 68Figure 4.2: Headbox Approach System o Twin-Wire Sheet Fomier 69Figure 4.3: Drainage Section o Twin-Wire Sheet Former 69Figure 4.4: Photograph ofTwin-Wire Sheet Former 71Figure 4.5: Photograph o a paper sheet produced 71


12/112

Figure 4.6: Additive locations and residence times on the TWF 73Figure 4.7: Water drainage along the TWF for three duplicate runs 76Figure 4.8: Filler retention in the wet web for three duplicate runs on the TWF 77Figure 4.9: Influence of cPAM on PCC retenti on on the TWF 78Figure 4.10: The effect of cP AM on AKD retenti on 8

l


13/112

LISTO T BLES

Table 1 1: Typical Concentrations and Sizes ofPapermaking Chemicalsfor the Production ofFine Papers 7

Table 2 1: Emulsion Characterization 21Table 2 2: Electrophoretic Mobilities and Partic e Diameters ofAKD Emulsions 38

~

Xl


14/112

CHAPTER

INTRODUCTION


15/112

1 1 BackgroundPaper producers are continually searching for new and more effective ways to

utilize chemicals and reduce operating costs without compromising paper quality.Since the operating speeds and manufacturing volumes of paper machines areconstantly increasing, the challenge to produce paper under such conditions is highlyreliant on additives. In the wet-end section of a paper machine, many chemicals areneeded to enhance the properties ofth paper and to improve its quality. Among thesechemicals are sizing agents, which produce hydrophobicity in paper. o study thebehavior of alkyl ketene dimer (AKD), a common sizing agent, we examine itschemical interactions in the forming section and short-circulation whitewater loop ofa paper machine. In this study, we focus on the processes and chemicals used tomanufacture fine office paper from kraft pulp fibers.

The papermaking chemicals studied here are added to the approach system ofthe paper machine (Figure 1.1). The approach system consists of a fan-pump loopwhere the di lute pulp suspension is metered, diluted, and additives are added. Flowenters the machine chest and moves to the stuff box , which regulates the feed byensuring a constant head. A valve before the fan pump controls the basis weightentering the paper machine. Both the flow from the machine chest, and whitewaterretuming from the wire pit, enter the fan pump. The fan pump is the Iargest pump inthe paper machine system, and dictates much of the shear delivered onto the papermachine. Next, the suspension is carried to the centrifugaI c1eaners and pressurescreens, before it flows into the pressurized headbox and onto the paper machine [1].

2


16/112

CENTRIFUGALCLEANERS

MACHINECHEST

PRESSURESCREENS

STUFFBOX

WlREHEAD BOX

WIREPIT

BASIS WEIGHTVALVE

Figure 1 1: Typical Approach System of a Paper Machine (adapted from Smook [1] )

Since paper mills are being urged to close their whitewater loops, more of thedissolved and colloidal substances DeS) or anionic trash , such as fines and otherdebris, are retuming to the headbox in the short-circulation whitewater loop. As aresult, the properties of the water being fed into the paper machine are altered due tothe elevated levels of electrolytes. This has a severe impact on the efficiency of otherchemicals added to enhance the quality and properties of the paper. Several studieshave shown that the behavior of additives is determined, in part, by the properties ofthe water. Since much of this water input into the paper machine now cornes from thewhitewater, t is important to know how these extra substances affect the waterchemistry [2, 3].

3


17/112

Closure o the whitewater cycle releases less water effluent to theenvironment consequently reducing the costs associated with the treatment o sucheffluent. However reusing this water leads to problems within the paper machinesuch as corrosion o the machinery due to high salt concentrations [4]. Another reasonto close the whitewater system is to reduce the loss o fibers which are valuable dueto the costs associated with processing them. The whitewater will always contain afraction o fibers so the volume lost should be minimized. t has also been shown thata build-up o electrolytes results in increased conductivity anionic trash contentsticky deposition and cationic polymer demand and has an adverse effect on paperquality [5] For example a build-up o negatively charged DCS can degrade theperformance o cationic retention-aid systems [6].

The decrease in first-pass retenti on o sizing agents results in paper that is lesssized. This has led to an increased use o sizing agents and retention aids to offset theeffects o DCS. Though this may be a temporary remedy the long-term solutionrequires an understanding o how to increase the first-pass retention o thesechemicals to optimize their use. The addition sequence and residence times oadditives employed in paper mills have been selected from previous experience andknowledge. However comprehension o the chemical reactions remains crucial in acomplete understanding o optimization.

Single-pass retenti on may be one o the most important factors influencingpaper quality and paper machine operation [1]. Many chemicals become less effectiveafter recirculation in the whitewater cycle. In fact it has been shown that insubsequent passes through the paper machine AKD contributes significantly less to


18/112

sizing than in the first pass. As mentioned later, the decrease in sizing is a direct resultof AKD reactions with water, which renders it non-reactive towards cellulose [7].

1 2 Objectives o the thesisSince AKD has a low first-pass retention ( ~ 4 0 ) , a large portion of it enters

the whitewater cycle and has ample time to interact with many types of particles,including dissolved and colloidal substances DCS) and precipitated calciumcarbonate PCC), a common filler. It is hypothesized that in the recirculation loopAKD may reverse its charge by adsorbing anionic material. When passing throughthe point of zero charge, AKD particles have the ability to aggregate with other AKDparticles. Other studies show that in the presence of water, AKD reacts to form a nonreactive ketone that does not contribute to sizing [8]. In addition, excessive usage ofAKD contaminates the whitewater system and may cause operating problems byforming deposits in the machinery [9]. However, it has not been shown whether thereexist any significant interactions between AKD and PCC, nor has any of this workbeen performed on a larger scale than in the laboratory. The experimental methodsemployed by all previous researchers involved the dynamic drainage j r or othersmall-scale methods.

Several theories exist to explain how AKD reacts with cellulose to form ahydrophobie monolayer in the paper structure. Extensive literature exists examining ifa covalent bond with cellulose is necessary for sizing [8, 10-13]. However, fewoutline the mechanism of AKD retention itself. This process occurs in the formingsection of papermaking and is critical to the subsequent AKD reaction with cellulose.


19/112

The objectives ofthis work are three-fold:(1) To detennine the conditions and time-scale under which AKD and PCC will

heteroflocculate, and relate these findings to the behavior of AKD and PCC inthe whitewater cycle;

(2) To study the behavior of AKD under wet-end conditions; in particular, todetennine the influence of PCC on non-retained AKD, and to understand themechanisms behind AKD s retention and its interactions with PCC and cationicpolyacrylamide (cP AM);

(3) To study retention of AKD on a laboratory twin -wire sheet fonner to acquire amore accurate interpretation of real interactions on a paper machine and todetennine the differences in a first-pass versus second-pass retenti on value.

A discussion of these objectives comprises the contents of the three subsequentchapters of this thesis. The remainder of this chapter presents a relevant literaturereview. An overview of wet-end operations in a papennaking machine is discussed,followed by a survey of AKD properties and interactions. In addition, PCC andcP AM data are provided to help understand the wet-end system as a whole. Chapter 2presents the interactions between AKD and PCC investigated on a small scale.Chapter 3 covers the interactions of AKD with cationic polyacrylamide. Chapter 4discusses the experiments perfonned on the laboratory twin-wire sheet fonnermachine. Finally, Chapter 5 concludes with a recapitulation of results andrecommendations for future work.

6


20/112

1 3 Literature Review1 3 1 Major Chemical Components

In the wet-end section of a paper machine, many chemicals are added tocontrol properties and improve the quality of the paper. Table 1 1 provides asummary of several chemicals used in the production of fine kraft paper, listed withtheir typical concentrations and average sizes.

Table 1 1: Typical Concentrations and Sizes ofPapermaking Chemicals for theProduction of Fine Pa ersComponent

Bleached SoftwoodKraft (BSK) Pulpstarch

w/w ofm ss0.5-1.00.03-1.00

Particle Diameter30flm; (up to3 mm length)

~ 8 0 1 0 0 nm' - - ' - ' ~ ' ~ ' ~ ~ ' - - ' - ' - - ~ ~ ~ ~ ~ ~ - ~ - - - ~ - - - - I5-10


21/112

1.3.1.1 Alkyl ketene dimer (AKD)AKD has been used as a sizing agent (to produce hydrophobicity in paper)

since the 1950's [14]. It is composed oftwo long carbohydrate chains, ranging from14 to 8 carbon molecules each, and a lactone ring in the center. AKD binds tocellulose fibers via a p-esterification reaction (Figure 1.2) [10]. This p-ester covalentbond was found to be vital for effective sizing [8]. The two hydrophobic tails form ahydrophobic layer on top of the sheet, retarding water penetration.

H~ C H = H R zo clo

R CH:z c C H C Ot l lo 0

Figure 1.2: AKD p-esterification with cellulose

AKD forms a stable colloidal emulsion in the presence of stabilizingpolymers, such as cationic starch. This gives the emulsion an overall positive charge,enabling AKD particles to adsorb onto the negatively charged fibers. Often, anotherpolymer with the opposite charge is added to give the emulsion amphotericproperties. AKD emulsions are stored with a dry solids content of 6-15 . AKDparticle sizes are in the range 0.2-2 microns, and addition levels are equivalent to0.05-0.2 ofpure AKD based on fiber [14].

The emulsion is typically kept at low pH and cooler temperature (+3C) toprevent hydrolysis. Under these conditions, it can remain stable for several months.As the pH increases, AKD begins to react with water (Figure 1.3). At a neutral pH,AKD is hydrolyzed quickly (4 AKD/hr) and at a basic pH of 10.4, it is completely

8


22/112

hydrolyzed in less than a day [5]. AKD hydrolysis is affected by pH, as well astemperature. Using NMR analysis, AKD was found to hydrolyze to its ketone formpalmitone) in a short period. More importantly, it was found that this ketone does not

contribute to sizing, since its ketone bond makes it un-reactive [8]. This ketone hasalso been found to cause deposition problems in paper machines [5].

Figure 1.3: Hydrolysis of AKD to palmitone

The acceleration in AKD hydrolysis can be attributed to CO{ and HC 3-ions. One the ory not fully developed) states that PCC accelerates hydrolysis bydestabilizing the AKD. Other theories describe AKD spreading on the PCC surface,or thirdly, sorne claim that the higher pH inside the PCC due to Ca OH)z ) catalyzesthe hydrolysis of AKD [5]. One study shows that AKD molecules adsorbed onprecipitated calcium carbonate PCC) filler convert to palmitone [15]. According to asecond researcher, AKD adsorbs to filler due to its high surface area [9]. Anotherstates that it is the high affinity ofAKD to filler responsible for AKD degradation [8].Other tests indicate there is little or no interaction between the filler and AKDparticles at a l [16]. With these contradictory results, it is difficult to draw any realconclusions.

To counteract the negative effects, increasing the AKD PCC ratio was foundto reduce the rate and extent ofAKD hydrolysis. For this reason, t has become thetrend to increase the quantity ofAKD in mill operations.

9


23/112

t is critical to maximize the first-pass retenti on of AKD in order to optimizesizing. Intermediate reactions interfere with AKD depositing onto the fibers, in amanner that reduces hydrophobicity. Solvent extraction tests indicate that sizingoccurs only when there is a covalent bond between the size molecule and cellulose[8]. This link is necessary to ensure the proper alignment of the molecule so thehydrophobic tails are exposed to the air interface [17]. Furthermore, it may bepossible that the bond serves as an anchor to prevent evaporation and detachment ofthe molecule at higher temperatures. In general, AKD retention is a dynamic processthat involves attachment, detachment, and charge neutralization and reversaI [16].

1 3 .1.2 Precipitated Calcium Carbonate (PCC)Precipitated calcium carbonate has become a popular filler since the change to

neutral or alkaline papermaking system was implemented. PCC is an example of amineraI filler that is added to the fumish to improve the optical and physicalproperties of the sheet. The particles fill the spaces between the fibers, resulting in adenser and smoother sheet. Since fillers are less expensive than fibers, they alsodecrease the overall cost of paper manufacturing [1]. However, fillers weaken thepaper strength by interfering with fiber-fiber bonding and by transferring polymerfrom fiber to filler surfaces [18]. Therefore, the amount of filler added to the sheet islimited by the resulting reduction in strength and sizing quality. Most fine paperscontain anywhere between 5-25 wt filler/wt fibers.

Since PCC is soluble at lower pH, it can only be used in a neutral or alkalineenvironment. CaC 3 is partially soluble in water and the Ca2 ions preferentially

10


24/112


25/112

deposits and improve operations [6]. In the production of fine kraft paper using PCC,cationic polyacrylamide (cP AM) is the most common retenti on aid. Cationicpolyacrylamide is an organic compound with a linear structure and a molecularweight in the range of3 6xl06 Da.

Highly charged cationic polyelectrolytes promote deposition of PCC on fibersby charge modification, while ones with a high molecular weight and low chargedensity act by a bridging mechanism [23]. In this study, we use cPAM with a degreeof substitution of 20% and a high molecular weight, indicating a bridging polymer.

The addition of a retenti on aid has a dramatic effect on the retention ofprecipitated calcium carbonate. Since both contaminated PCC and fibers have a netnegative charge, they tend to repel each other. Cationic polyacrylamide imparts anattraction (via bridging), thereby improving PCC retention. t is important todetermine the optimal quantity of cP AM to add since quantities that are too highhinder sheet formation [6].

1.3.1.4 FibersCellulose is negative1y charged. The negative charge originates from the

carboxyl groups that dissociate to various extents at different pHs. Under normalpapermaking conditions ( ~ p H 7), the carboxyl and sulphonic acid groups are themajor contributors to the fiber charge [24].

A fraction ( ~ 1 O - 2 0 ) of the fiber mass is comprised of fines. By definition,these are the smaller segments of fibers whose diameters are less than 76 microns.

l Contaminated PCC refers to the particles in process or tap water whose charge has been influencedby impurities in the water.

2


26/112

Fines have a higher specific surface area ~ 1 0 m2jg [25] than fibers 1 m2jg , whichgives them the capacity to adsorb much more material per unit surface area. Becausethey are smaller than the fabric aperture, fines commonly drain through without beingretained in the sheet, taking sorne o the additives with them.

1 3 2 Retention MechanismsAKD retenti on can be improved by either physical or chemical methods. For

example, Mattsson [26] suggests that AKD retention can be improved physically byadding a chemical to induce AKD molecules to aggregate prior to addition into theapproach system. This method o preflocculation produces aggregates that cangrow to a size up to 30 microns in diameter. In such a way, the AKD flocs areretained in the sheet since they are less likely to pass through the wire. Testsdemonstrated that on a laboratory scale (employing the Britt Dynamic Drainage Jar),AKD molecules that formed aggregates had a significant improvement in single-passretention. However, the question arose whether these aggregates would not cause anuneven distribution in sizing on the paper sheet [26].

Instead, we consider the chemical interactions that occur between theadditives and fibers that allow us to optimize the retention. By understandingtheinteractions that occur, we can predict the effects o changing the parameters. Theintent o this study is to find the fundamental mechanisms responsible for theinteractions.

13


27/112

1 4 References[1] Smook, G.A., Handbook for Pulp Paper Technologists , Angus WildePublications, Vancouver, 2nd Ed., (1992).[2] Donat, V., van de Ven, T.G.M. and Paris, J., Distribution o dissolved and

colloidal substances in the forming and press sections o a paper machine ,Journal ofPulp and Paper Science 29(9) 294-298 (2003).[3] Huber, P., Carre, B., Mauret, E and Roux, J.-C., The influence o fine

elements build-up in the short-circulation on fibre flocculation. Preprints -International Paper and Coating Chemistry Symposium, 5th, Montreal, QC,Canada, (2003).[4] Gavelin, G., Paper Machine Design and Operation , Angus WildePublications, Vancouver, (1998).[5] Jiang, H. and Deng, Y., The effects ofinorganic salts and precipitatedcalcium carbonate filler on the hydrolysis kinetics o alkyl ketene dimer ,Journal of ulp and Paper Science 26(6) 208-213 (2000).[6] Allen, L.H., Polverari, M., Levesque, B and Francis, W., Effects o systemc10sure on retention- and drainage-aid performance in TMP newsprintmanufacture , Tappi Journal 188-195 (1999).[7] Mattsson, R., Sterte, J. and Odberg, L. Colloidal Stability o AKDdispersions. in The Science ofPapermaking: Transactions o the 12thFundamental Research Symposium Ed.Baker.Oxford, UK, (2001). 393-415.[8] Bottorff, K.J., AKD sizing mechanism: A more definitive description , TappiJournal 77(4) 105-16 (1994).[9] Esser, A. and Ettl, R. On the mechanism o sizing with alkyl ketene dimer(AKD): physico-chemical aspects o AKD retenti on and sizing efficiency. inFundamentals ofPapermaking Materials, Transactions o the 11th

Fundamental Research Symposium Cambridge, UK, (1997). 997-1020.[10] Bottorff, K.J. and Sullivan, M.J., New insights into the alkylketene dimer(AKD) sizing mechanism , Nordic Pulp Paper Research Journal 8(1) 86-95 (1993).[11] Isogai, A., Effect o cationic polymer addition on retenti on o alkylketenedimer , Journal of ulp and Paper Science 23(6) 276-281 (1997).

[12] Isogai, A., Stability o AKD-Cellulose B-Ketoester Bonds to VariousTreatments , Journal ofPulp and Paper Science 26(9) 330-334 (2000).[13] Lindstrom, T and Soderberg, G., On the mechanism ofsizing with alkylketene dimers, part 1 Studies on the amount o alkyl-ketene dimer requiredforsizing different pulps , Nordic Pulp and Paper Research Journal 1(1) 26-33 (1986).[14] Roberts, J.C. Neutral and Alkaline Sizing in Paper Chemistry Ed.Roberts,J.C. New York: Blackie Chapman Hall, (1991).[15] Scott, W.E., Wet End Chemistry , Tappi Press, Atlanta, (1996).[16] Champ, S and Ettl, R., The dynamics o Alkyl Ketene Dimer (AKD)retention. Preprints -5th International Paper and Coating ChemistrySymposium, Montreal, QC, Canada, 285-291 (2003).

14


28/112

[17] Yu, L. and Garnier, G., The role ofvapour deposition during internaI sizing:a comparative study between S and AKD , Journal ofPuip nd PaperScience 28(10) 327-331 (2002).

[18] Cho, B.-U., Garnier, G. and van de Ven, T.G.M., Parameters affecting paperformation on a pilot fourdrinier using cP AMlbentonite retenti on aids ,Preprints -5th International Paper and Coating Chemistry Symposium,Montreal, QC, Canada, 193-200 (2003).

[19] Vanerek, A., Alince, B. and Van De Ven, T.G.M., Colloidal behavior ofground and precipitated calcium carbonate fillers: effects of cationicpolyelectrolytes and water quality , Journal ofPulp nd Paper Science 26(4)135-139 (2000).

[20] Mitsui, K., Mechanism of Fines Retention and Drainage with aPolyacrilamide/Bentonite Retention Aid , thesis, Chemical Engineering,McGill University, (2000).

[21] Alince, B., Time factor in pigment retention , Tappi Journal 79(3) 291-294(1996).[22] Tarn Doo, P.A., Kerekes, R.J. and Pelton, R., Estimates of MaximumHydrodynamic Shear Stresses on Fibre Surfaces in Paper machine Wet End

Flows and in Laboratory Drainage Testers , Journal ofPulp nd PaperScience 10(4) 80-88 (1984).[23] Vanerek, A., Alince, B. and van De Ven, T.G.M., Interaction of calcium

carbonate fillers with pulp fibers: effect of surface charge and cationicpolyelectrolytes , Journal ofPuip nd Paper Science 26(9) 317-322 (2000).

[24] Lindstrom, T. Electrokinetics of the Papermaking Industry in PaperChemistry Ed.Roberts, J.C.New York: Blackie Chapman Hall, (1991).[25] Porubska, J., Microstructure and properties ofTMP papers , M.Eng. thesis,

Chemical Engineering, McGill University, (2000).[26] Mattsson, R., Sterte, J and Odberg, L., Sizing with pre-flocculated alkylketene dimer (AKD) dispersions , Nordic Pulp Paper Research Journal17(3) 240-245 (2002).

15


29/112

CHAPTER

AKD AND PCC INTERACTIONS


30/112

2 1 Abstract

Experiments with AKD and PCC were performed on a laboratory scale usingthe Photometrie Dispersion Analyzer PDA). What was originally thought to eheteroflocculation between PCC and AKD was found to be PCC homoflocculation bystarch, the AKD stabilizer. SEM photographs show that PCC and AKD eachhomoflocculate; little to no heteroflocculation occurs. With the results from PDAtests, this suggests that PCC and AKD do not directly interact. Instead, starch removalfrom the AKD particle surface aids the homoflocculation of PCC via polymerbridging). Moreover, this starch transfer results in AKD particle instability, causingthe AKD particles to aggregate since they have been depleted of their stabilizingpolymer. Therefore, t can also be concluded that cationic starch has a higher affinityfor PCC than for AKD.

More importantly, we find that AKD and PCC are not likely to interact in thewhitewater cycle. Since no heteroflocculation occurs, this dismisses the idea that PCCwi111 wer AKD single-pass retention.

17


31/112

2 2 Introduction

Extensive work has been done to investigate AKD and PCC interactions inwet-end papermaking though there are few conclusive results. It is stated in severalpapers that PCC causes AKD hydrolysis leading to a great loss o sizing and causingsize reversion after drying [1-3]. However the mechanism for these interactions andthe conditions under which they occur is unknown. Furthermore no research has beendone to determine whether PCC affects the first-pass/second-pass retention o AKD.Recirculation o AKD in the whitewater cycle may be detrimental to its sizingefficiency. n this chapter we explore i this is a direct effect o PCC.

The chemical interactions that occur in the wet-end depend on the order inwhich chemicals are added as well as their residence time prior to the headbox.Typically PCC is added near the first mix box overflow and AKD is added next inthe second mix box [4]. Thus AKD is likely to interact with PCC and fibers. Theinteractions depend on the state o flocculation and the state o the suspension; that iswhether PCC has adsorbed to the fibers or formed flocs with the polymeric retentionaid. Several theories suggest what will occur upon addition o AKD. Many articlesdiscuss the interactions between AKD and fibers AKD and PCC or cP AM or howone affects the other but none o this research has been done on a wet-end papermachine environment or an industrial time scale [5-8]. In this chapter we presentresults from the small-scale trials conducted to investigate the behavior o PCC andAKD in similar laboratory environments. In Chapter 4 trials with AKD and PCC areperformed on a larger-scale using the laboratory twin-wire sheet former.

8


32/112

2 3 Emulsification o AKD Stabilized with Cationic Starch

Most large-scale AKD emulsion preparations consist o additionalcomponents, such as surfactants, polymers, stabilizers, alcohols, etc. However, detailso the commercial formulations are usually not disclosed. Therefore, the researchercannot know i interactions o an emulsion are due to AKD or other chemicals withinthe emulsion. To solve this problem, we developed a method to create a simple butstable AKD emulsion in our laboratory. The emulsion consisted o only AKD and astabilizer cationic starch). To design our method for emulsification, we referencedseveral other researchers who produced their own AKD emulsions and developedmethods to properly analyze its properties [9, 10].

As mentioned in the previous chapter, alkyl ketene dimer AKD) is a waxyester with two fatty chains, typically ranging in length from 4 to 8 carbonmolecules joined by a lactone group. t is extremely hydrophobic and insoluble inwater. Upon addition to fibers, the AKD can react with cellulose to form a f3-estercovalent bond. However, this is a slow reaction and sizing only begins in the dryingsection o a paper machine, continuing throughout the drying and curing process forabout a week afterwards [11].

For AKD to remain in emulsion, a stabilizer must be added to prevent theAKD drop lets from coalescing during emulsification. Since AKD has a melting pointo about 500 e depending on chain length), the emulsification is done at a highertemperature so that the AKD is in its molten phase. Immediately after emulsification,the solution is quenched below room temperature for the AKD to revert to a morestable solid.

9


33/112

2 3 1 Materials and MethodsAlkyl ketene dimer (from Raisio Roe Lee) was obtained in pellet form. A 10 g

sample was melted to 60C in preparation for sonication. A cationic (quatemaryamine-substituted) starch CATO-237 was obtained from National Starch in a powderform. According to the manufacturer, this chemical has a nitrogen content ( N) of0.36-0.44. This value can be converted using the following formula (2.1), whichequates to a degree of substitution of 0.043-0.053 . The degree of substitutionindicates the percentage of amine groups on the starch with a positive charge. Thistranslates into the overall cationic charge.

DS = 162x N1400 - (152x N) (2.1)The starch was dispersed in de-ionized water to either 1 , 2 or 4 starch

concentration. t was cooked for 40 minutes in a 93 oC water bath, as suggested by themanufacturer. This hot starch solution was added to a 100 mL beaker containing themeltedAKD.

A sonicator from Vibracell (VCF-1500, Sonics Materials, Inc) was used foraIl emulsifications. This apparatus operates at a frequency of 20 kHz. The tip of thesonicator, which is one inch in diameter, was inserted into the liquid. The sample wassonicated for three cycles of three minutes each to ensure proper homogenization.

Following sonication, the solution was immediately quenched in a metal beakercontaining 100 mL of cold de-ionized water. Hydrochloric acid lM) was added toreduce the pH to approximately 4 Samples were kept refrigerated until furthertesting. The composition of this emulsion is presented in Table 2.1, showing adispersion that has 53.4 mg AKD/mL solution.

20


34/112

Table 2.1 Emulsion Characterizationo m ~ o n e n t u a n t i t ~

AKD g) 1Starch solution g) 1.5Starch solution volume ml) 75De-ionized water ml) 1001M HCI ml) 0.5Total Volume ml) 187.2[AKD] mg AKD/ml) 53.4

2.4 Emulsion Analysis2 4 1 Particle Size Analysis

The Malvem Mastersizer2000 (Malvem Instruments, UK) was used to test theAKD particle size as well as particle-size distribution. This technology relies on laserdiffraction, using the Mie theory to determine the particle-size distribution. Particles

are passed through a focused laser beam, which causes them to scatter light. Theintensity of this light is measured as a function of the scattering angle y multiplephotosensitive detectors. The refractive index of AKD was taken as 1.485 (a typicalvalue of a wax) in water with a refractive index of 1.33. Plots such as the one seen inFigure 2 1 were obtained. The particle diameter ranges from approximately 0.2 /lm to5 /lm It is normal to expect a variation in particle sizes. For the precision requiredhere, the emulsion is relatively monodisperse. The calculated median value (byvolume) ofthe samples was used as the average particle diameter.

21


35/112

12,.-.. 10

8lE 6:l'0> 42

~ 0 1 0 1 10 100Partiele diameter ~ m )

Figure 2.1: Particle-size distribution o AKD emulsion (Malvem Mastersizer)

Periodic testing revealed that the refrigerated emulsions were stable for atleast two weeks. After four weeks, the two phases (AKD and water) began toseparate. Since AKD has a lower density than water, it rises to the top, causingcreaming. This instability can be due to creaming by coalescence, aggregation, orboth1 [9] AKD stabilized with the highest DS starch was found to be most stable.

The particle size was confirmed with the Acoustic ElectroacousticSpectrometer (Dispersion Technology, Inc., NY). This apparatus calculates theparticle size distribution by minimizing the deviation between the measured andcalculated acoustic attenuation spectra. This method also gave a slightly higher valueo 1 27O 59 Ilm

In addition to particle size analysis, photographs were taken by scannmgelectron microscopy (SEM). Samples were prepared by diluting the emulsion andplacing a drop on an SEM pin. When the drop let had dried, it was sputter coated with

1 Coalescence: fusion o individual particles into one greater particleAggregation: coming together o individual particles to form a cluster, particles not evenly dispersed

22


36/112

a gold film. Pictures were taken at 5,OOOX and 1O,OOOX magnification. As seen inFigure 2.2, these photographs reveal a homogeneous sampling of AKD drop lets thatare less than a micron in diameter. Although most partic1es are similar in size, severaloutlying drops that have likely coalesced also appear. t is interesting to note that thepartic1es are not perfectly spherical. We can only presume that this is due to the SEMcoating process, when the sample is exposed to extreme vacuum conditions.

Figure 2.2: SEM image ofAKD emulsion at 1O,OOOX magnification scale 1 J.lm

2 4 2 Electrophoretic Mobility AnalysisElectrophoretic mobility EM) is a measure of the partic1e charge. This

mobility value indicates the strength of electrostatic attractions or repulsions) apartic1e experiences with another charged partic1e in proximity. AlI tests were done inboth de-ionized DI) and tap water. A di lute sample ofth AKD emulsion was pouredinto a flat cell equipped with an electrode at either end. A voltage was applied and thevelocity of partic1es at a specifie distance from the tube wall could be measured on amagnified screen divided into grids. By knowing the time, t for a partic1e to travel

23


37/112


38/112

2 4 3 Effect ofStarch Concentration nd RatioSeveral variations of the emulsification procedure were tested. One variable

that was modified was the ratio between AKD and starch. From the literature, asuggested range for this ratio is 0.1-0.3 1 g AKD for each 0.1-0.3 g starch) [9].Several trials were done with various ratios; it was found that 19 AKD: 0.15 g starchconsistently produced the same partic1e diameter, and therefore this ratio was used inall subsequent emulsions.

Various starch concentrations were tried as well. Using starch cooked at a4concentration, the AKD particle sizes tend towards 2.3Jlm diameter. As the starchconcentration decreased, the partic1e size decreased as well. Using a 1 starchsolution, we obtained AKD droplets that were nearly identical to the theoreticalvalues (Figure 2.3).

4 - - - - - - - - - - - - -- - - - - - - - - - - - -- - - - - - - - - -

0

4 starcho 2 starch.. 1 starch

- Theoretical Value

o L ~ = = : = = = ~ ~ ~ ~ ~ J..0.05 0.10 0.15 0.20 0.25 0.30 0.35 0.40

C (Starch:AKO Ratio)Figure 2.3: Theoretical and experimental values of AKD partic1e diameter

Theoretically, we can predict the smallest stable AKD partic1e size that can occur forvarious ratios using the following formula, where C is the ratio, a is the radius of

25


39/112

AKD droplets, is the density o AKD wax, and r max is the maximum surfacecapacity o starch, which is taken from the literature as lOg/m [13].

(2.3)

Solving for and converting to a diameter, we obtain the line shown in Figure 2.3. Asthe starch concentration approaches 1 , the experimental particle size approaches thetheoretical size.

t was determined that concentrated solutions o starch produced larger AKDdroplets. As the starch concentration was decreased, the AKD particles becamesmaller after receiving the same sonication treatment. These size variations wereattributed to starch clustering. This topic has been covered in depth by severalresearchers in the area o pulp and paper, including Shirazi [13, 14]. After sorneexperiments, we found that starch clustering is sensitive to sonication. At higher

starch concentrations, sonication is not able to disperse the starch completely;however, it is much simpler with low concentration solutions. Thus, in theemulsification with 4 starch solutions, the starch was not able to completely un-cluster, so fewer starch clusters were available to coat the AKD droplets. To remainstable, the AKD droplets remained larger to decrease their specific surface area. Inthe 1 starch solution, the starch was completely un-clustered, making each starchmolecule available for coating the AKD, thus producing smaller drop lets. A standardradius o gyration for starch in literature is given as 108 nm [13]. With the di lutesolution, we come close to achieving this value (see Appendix B for a review ostarch clustering as a function o solution concentration and sonication). When our

26


40/112

results are represented graphically as in Figure 2.3, we can demonstrate that the AKDdrop lets are more dependent on the concentration o starch than the actual proportionso AKD and starch.

In the end, we chose to use a 2 starch solution, and consistently obtained aparticle diameter o 0.8 microns. These emulsions were then further used for allPhotometrie Dispersion Analyzer experiments, adsorption kinetics, and experimentson the laboratory twin-wire sheet former.

2 4 4 Camparisan t Cammercial EmulsianA sample o a commercial emulsion was also tested for particle size with the

Malvem Mastersizer. Under the same water and pH conditions, its average particlesize was 0.45 microns in diameter. However, its electrophoretic mobility was higher,at +1.8 [.un/s)/(V/cm)]. In comparison, our AKD emulsion had a slightly largerdiameter and a weaker charge, but the values were similar, and we concluded that theemulsions were comparable.

2.5 Photometrie Dispersion Analyzer ExperimentsTo study AKD and PCC interactions in a wet-end environment, we began

observations at the laboratory scale, with experiments using the PhotometrieDispersion Analyzer (PDA) (Rank Brothers Ltd., UK). In typical papermakingoperations, the proportions o PCC and AKD added are about 10-15 wt PCC/wtfiber, and 0.15 wt AKD/wt fiber; thus the ratio o these two additives would beapproximately 10-15 mg AKD/ g PCC.

7


41/112

The Photometrie Dispersion Analyzer provides a sensitive but qualitative)indication of the changes in the state of aggregation of a suspension. A more in-depthdiscussion of the princip les behind the PDA operation is given in Appendix B.

2 5 1 Methods nd Materials

Precipitated calcium carbonate PCC) Albacar HO Specialty MineraIs Inc.)was used in aIl experiments. This PCC has an average partic1e diameter of 1.3 ~ anda high specifie surface area of ~ 2 m2/g, due to its scalenohedral structure. Thiscompound and its behavior with fibers and other chemicals has been fullycharacterized elsewhere [15, 6, 16, 7 8].

The AKD emulsion was created in our laboratory; its properties were outlinedat the beginning of this chapter. AIl experiments were performed in tap water with aconductivity of280 ~ S / c m which resembles industrial waters [17, 18].

A 5g/100mL stock solution of precipitated calcium carbonate was prepared intap water. A portion was measured and added to a one-liter solution of tap water toproduce a 500ppm suspension of PCc The concentration of PCC was held constantbetween experiments, while the quantity ofAKD varied. AKD was added 30 secondsafter the start of each experiment. Tubing of 3mm I.D. was used for aIl trials tominimize the effect of shear and to allow a larger flowrate and floc size. Operating ata higher shear rate may break up flocs and narrower tubing would give an incorrectindication of flocculation rates. The suspension was circulated at 150 mL/min, andstirred at 100 rpm in the beaker. Figure 2.4 shows a schematic ofthis setup.

28


42/112


43/112


44/112

We chose intennediate values for both; 100rpm and 150 mL/min flowrate.This is equivalent to a relatively low shear rate o approximately 20 S-1 in the beaker.Shear is greater in the tubing 650 S-I) due to the small tube diameter.

1.6 Flowrate: [mL min]1.4 1501.2

::::> 1.0 0.80+=co 0.6

0.4

0.2

0.00 100 200 300 400 500 600 700

time 8)Figure 2.6a: Effect ofvarying flowrate on the floc fonnation in PDAfor a stirring rate o 150 rpm.

31


45/112

1.61.41.2

> 1.0C- 0.80CO 0.6::

100 rpm0.40.20.0

0 100 200 300 400 500 600 700tim (8)

Figure 2.6b: Effect ofvarying stirring speed in beaker on the floc formationin PDA for a flowrate of 150 mL/min

2 5 3 Results Discussion

2.5.3.1 PCC HomoflocculationWhen the signal from PCC in the water was measured without AKD present, a

small increase in signal was detected. This accounts for the minimal flocculation thatoccurs since positively-charged PCC acquires a negative charge in tap water.Nevertheless, no significant flocculation occurs under these conditions. Figure 2.7shows this trend as the curve labeled 0 mg .

2.5.3.2 PCC and AKD FlocculationTests performed with PCC and various additions of AKD emulsion showed

that there was a maximum in flocculation that occurred at 1 mg AKD/g PCC (Figure2.7). (As stated earlier, this is in the range of typical addition levels in industrial

32


46/112

settings.) In all PDA figures, the left axis represents the RATIO o the root-meansquare/ DC signal, in arbitrary units based on the gain settings o the PDA). Thisvalue indicates the relative floc size. For larger flocs, there are less flocs for the samenumber o particles initially, resulting in a larger RATIO value. Note that themaximum ratio in these tests reaches 1.2. All additions o AKD are per gram o PCC.

Below 1 mg AKD/g PCC, flocculation was slower and reached a lower finalvalue. At higher AKD dosages, the initial slope was maximum, the same as that for1 mg/g PCC), but flocculation reaches a lower plateau. Tests were repeated to verifythat this behavior is reproducible.

Since PCC is white with a high refractive index, the PDA signal primarilyreflects the aggregation o PCC. That is, we have no real evidence that AKD and PCChave heteroflocculated, but rather that PCC has been flocculated. This is particularlyinteresting since we discovered later that the flocculation inferred by the PDAconsisted o only clusters o PCC and starch molecules. This was confirmed withexperiments testing the flocculation o PCC by cationic starch, and likewise, by AKDfree o starch.

The results shown below suggest that there exists an optimal AKD additiondosage, at which half o the PCC is covered by starch), allowing for maximumflocculation. Beyond this, the PCC becomes over-coated, leading to a quick initialflocculation, but the flocs cannot continue to grow since they are fully coated. Figure2.7 shows this phenomenon with the 50 mg line. Although restabilization may beexpected in these conditions, van der Waals forces are able to form weak bondsbetween fully-coated PCC particles, allowing them to homoflocculate.

33


47/112

->~ 0;cac:::

1.4

1.2

1.0

0.8

0.6

0.4

0.2' _ ~ I W ~_ 0 mg

l . . A ' ' - ~

0.00 100 200 3 400 500 600 700

time s)Figure 2.7: PCC flocculation with AKD emulsion. Quantities indicated areAKD dosages [mg KDIg PCC].

2.5.2.3 PCC and Starch FlocculationFigures 2.8 and 2.9 demonstrate the PCC flocculation with the addition o

cationic starch. The starch was sonicated to un-c1uster starch molecules. At lowadditions less than lmg starchlg PCC), the flocculation is slow, and reaches a plateauheight relative to the amount o starch added. Above this optimum dosage, the initialflocculation rate remains the same, but the flocs are not able to grow. This impliesthat for PCC fully coated by starch, no energy barrier is present in the interactionenergy between two partic1es. Thus, cationic starch do es not cause steric orelectrostatic stability, and its adsorption does not eliminate aggregation o PCC, sincevan der Waals forces are still present. However, the bond strength between fully-coated PCC partic1es is weaker than that for partially coated partic1es, as indicatedschematically in Figure 2.10.

34


48/112

>

o; ;ct:::

2.0 - r - - - - - - - - - - - - - - - - - - - - - - - - - - -

1.5

1.00.25 mg

Iane '.e.0.5 r cr 0.1 mg starch

0.0 U . l I I ~ - _ T ' - - - - , - - - . , _ _ - - _ . _ - - _ _ , _ - - - , _ _ - - - ;o 100 200 300 400 500 600 700time s)

Figure 2.8: PCC flocculation with sonicated starch low starch additions) Labelsindicate [mg starch/g PCC].

2.0 - r - - - - - - - - - - - - - - - - - - - - - - - - - - -

1.5-:j


49/112


50/112

filtered AKD solution AKD1 to obtain STARCH2 and AKD2 . A schematic ofthe filtering is presented in Figure 2.11.

AKD

l > 1n ~oTARCH 2 ) . J LJoTARCH 3

Figure 2.11: Successive filtration schematic ofAKD droplets and starch stabilizer

PDA experiments were performed on both the filtrate and the AKD droplets. Resultswith the filtered starch were identical to the ones obtained with sonicated starch in theprevious section. Experiments with the filtered AKD are described below.

Figure 2.12 shows that the AKD partic1es, now filtered and depleted ofcationic starch, hardly flocculate with PCC, even at high quantities such as 50 mgAKD/g PCC. In other words, upon a second filtration of the AKD emulsion, onlyminimal flocculation was observed, similar to the behavior of PCC alone.

The electrophoretic mobility of the filtered AKD was measured, and the valuewas found to be lower than that of the original emulsion (Table 2.2). This is expectedsince AKD itself carries no charge.

37


51/112

-:Jci0.6. ...----------------------

0.5


52/112


53/112

Figure 2.14 below presents an AKD emulsion at 1O 000X magnification. Wecan see the spherical AKD drop lets with diameters just under 1 micron. As mentionedbefore, we must assume that the indentations and irregularities in their sphericalshape are due to the process of coating at an extremely low pressure.

Figure 2.14: SEM image of AKD emulsion scale 1 micron)

Next, we added AKD and PCC in various proportions on a pm. Usingstandard additive quantities lg PCC and 10mg AKD), AKD appears 100 times 1essfrequently than PCC, and is not easily seen in the photographs. N everthe1ess, noevidence of AKD-PCC heteroflocculation was observed at these addition levelsoptimal additions).

Figure 2.15 a,b and c show PCC and AKD at a 1:1 ratio. Although this ishigher than standard quantities for AKD, we do not see evidence of AKD-PCCinteractions. We observe homofloccu1ation of both AKD and PCC, though there wereno interactions between the two species.

40


54/112

a

cFigure 2.15 a,b,c): PCC and AKD at a : ratio

4


55/112

Figure 2.16: SEM image o AKD emulsion after filtering out free starch; AKDdrop lets aggregate.

In Figure 2.16 we show an AKD emulsion where the starch has been filtered away.This photograph illustrates how the removal o starch results in the instability o theindividual AKD partic1es causing them to aggregate.

2 7 Conclusions

In this chapter we have outlined a procedure for creating an AKD emulsionfree from extraneous substances. Its properties were similar to those o a commercialemulsion and we proceeded to use it for further testing. U sing the PhotometrieDispersion Analyzer we found that the AKD emulsions flocculate with PCC at anoptimal dosage o 10mg AKD/g PCC. However later we found that flocculation isactually caused by the cationic starch acting as the AKD stabilizer.

No visual proof was found that AKD and PCC form flocs. The SEMphotographs show that PCC and AKD each homoflocculate; little to noheteroflocculation occurs. This in combination with the results obtained from the

42


56/112

PDA tests, suggests that PCC and AKD do not directly interact. Instead, starchremoval from the AKD particles aids the homoflocculation o PCC via polymerbridging). Starch transfer causes the AKD particles to aggregate since their stabilizingpolymer starch) has been depleted. t can therefore be concluded that cationic starchhas a higher affinity for PCC than for AKD.

t is interesting to consider what these findings imply. Previously, it wasthought that PCC directly hydrolyzes AKD. Now, we see evidence that AKD andPCC do not heteroflocculate, but that polymer transfer occurs from AKD to PCC,followed by PCC homoflocculation. This indicates that the alkaline environment isthe most significant factor contributing to AKD hydrolysis. More importantly, wefind that AKD and PCC are not likely to interact in the whitewater cycle. Thisdismisses the theory that PCC will lower AKD retention. PCC does decrease thesizing o a sheet by increasing the specifie surface area and forming more pores.However, this would not influence AKD single-pass retention.

2 8 AcknowledgementsMany thanks to Helen Campbell for assistance with the Scanning ElectronMicroscope at McGill University and to Lou Cusmich for help with the electronicdata acquisition setup for the PDA. 1 would also like to thank Patrick Lim Soo for hisassistance with the Dynamic Light Scattering.

4


57/112

2.9 References

[1] Bottorff, K.J., AKD sizing mechanism: A more definitive description , TappiJournal 77(4) 105-16 (1994).

[2] Bottorff, K.J. and Sullivan, M.J., New insights into the alkylketene dimer(AKD) sizing mechanism , Nordic Pulp Paper Research Journal 8(1) 86-95 (1993).[3] Scott, W.E., Wet End Chemistry , Tappi Press, Atlanta, (1996).[4] Brungardt, C.L., Studies indicate wet-end additive effects on AKD retention,

reaction efficiency , Pulp Paper 75(6) 47-49 (2001).[5] Isogai, A., Effect of cationic polymer addition on retention of alkylketene

dimer , Journal of ulp and Paper Science 23(6) 276-281 (1997).[6] Kamiti, M. and Van de Ven, T.G.M., Kinetics of deposition of calciumcarbonate particles onto pulp fibers , Journal of ulp and Paper Science

20(7) 199-205 (1994).[7] Vanerek, A., Alince, B. and van De Ven, T.G.M., Interaction of calciumcarbonate fillers with pulp fibers: effect of surface charge and cationic

polyelectrolytes , Journal ofPulp and Paper Science 26(9) 317-322 (2000).[8] Vanerek, A., Alince, B. and Van De Ven, T.G.M., Colloidal behavior ofground and precipitated calcium carbonate fillers: effects of cationicpolyelectrolytes and water quality , Journal ofPulp and Paper Science 26(4)135-139 (2000).[9] Chew, Y.S., Peng, G., Roberts, J.C., Xiao, H., Nurmi, K and Sundberg, K.,

Characterization of the stability of cationic starch stabilised AKDemulsions , Preprints - 5th International Paper and Coating ChemistrySymposium, Montreal, QC, Canada, 331-337 (2003).

[10] Mohlin, K., Leijon, H. and Holmberg, K., Spontaneous emulsification ofalkyl ketene dimer , Journal ofDispersion Science and Technology 22(6)569-581 (2001).

[11] Roberts, J.C. Neutral and Alkaline Sizing in Paper Chemistry Ed.Roberts,J.C. New York: Blackie Chapman Hall, (1991).[12] Esser, A. and Ettl, R. On the mechanism ofsizing with alkyl ketene dimer(AKD): physico-chemical aspects of AKD retenti on and sizing e f f i c i e n c y ~ inFundamentals of Papermaking Materials, Transactions of the Il thFundamental Research Symposium Cambridge, UK, (1997). 997-1020.

[13] Shirazi, M., Van de Ven, T.G.M. and Garnier, G., Adsorption of ModifiedStarches on Pulp Fibers , Langmuir 19(26) 10835-10842 (2003).

[14] Shirazi, M., Van de Ven, T.G.M. and Garnier, G., Adsorption of ModifiedStarches on Porous Glass , Langmuir 19(26) 10829-10834 (2003).[15] Cechova, M., Alince, B and van de Ven, T .G.M., Stability of ground and

precipitated CaC 3 suspensions in the presence of polyethylene oxide andkraft lignin , Colloids and Surfaces A: Physicochemical and EngineeringAspects 141(1) 153-160 (1998).[16] Suty, S., Alince, B. and van de Ven, T.G.M., Stability of Ground andPrecipitated CaC 3 Suspensions in the Presence of Polyethylenimine andSalt , Journal ofPulp and Paper Science 22(9) 321-326 (1996).

44


58/112


59/112


60/112

3 1 Abstract

Interactions o AKD with cP AM were examined using both anionic andcationic AKD emulsions. PDA experiments showed that cationic polyacrylamide didnot induce flocculation o cationic AKD (stabilized by cationic starch). Conversely,anionic AKD was found to flocculate by cP AM. Kinetic adsorption tests wereperformed to determine the rate o AKD deposition onto fibers under shearconditions.

We found that without addition o retention aid, 75 o cationic AKDparticles deposited onto fibers, with detachment beginning after a peak in deposition.With the addition o cP AM, the retention increased to 100 . Although cP AMmolecules do not flocculate AKD on their own, cP AM adsorbed to fibers is capableo adsorbing onto AKD by a mechanism known as asymmetric polymer bridging. Wepropose that the AKD-cP AM -fiber bond is stronger than the one between starchcoated AKD and fibers, explaining the complete retenti on and absence o particledetachment.

Anionic AKD emulsions have become common in industry. In testing theadsorption kinetics with this emulsion, we found that the absence o a retenti on aidresulted in very low AKD retention. Addition o cP AM rapidly increases the rate oAKD deposition onto the fibers even at high dosages, where we would expect stericrepulsions to limit the deposition. This is likely a kinetic effect, since the AKDdeposition onto fibers is faster than the coating o AKD particles by cP AM.

7


61/112

3 2 Introduction

n further studying the retention mechanism o AKD we investigated AKDinteractions with a cationic retenti on aid. t is not enough to consider only AKD andfiber interactions since in practice retention aids also interact with these compounds.

n a paper mill retention aids are added to improve fines retenti on increasedrainage control deposits improve filler retention as well as to improve operations[1]. To function properly the retention polymer must adsorb onto the surfaces o thefibers and the particles to be retained. The adsorption o cationic polymers ontoanionic surfaces such as cellulose fibers and clay occurs rapidly because there is nobarrier to adsorption. Polymers are usually added at a point close to the headbox tominimize disruption o the flocs and to maximize retention. t is believed that uponadsorption the conformation o the adsorbed polymer facilitates the formation opolymer bridges between surfaces promoting the retention o fines [2]. Changes inconformation o the polymer occur after adsorption usually making bridging lesseffective although fuis mechanism is not well understood.

t is concluded by one researcher that the improvement o AKD retenti on isnot caused by direct adsorption o retenti on aids on AKD particles but by AKDadsorption on fines and fibers leading to co-flocculation o fines fibers and AKDparticles [3]. Another study states that retention is thought to be caused byheterocoagulation o cationic AKD particles to negatively charged fiber surfaceswhere cationic polymers act as fixatives for AKD [3 4]. Using cPAM one studyshowed that AKD deposition reached a maximum after 20 seconds indicating thatAKD retention is a reversible process. This observation suggests that the subsequent

48


62/112

decrease in AKD deposition is due to particle detachment/charge reversaI, indicatingstarch transfer. The results proved that AKD retention is optimized if the contact timebetween AKD and fibres is short, and that of AKD-fibres and cationic polymers isshort as well [4]. The absence of re-attachment was explained by the deactivation ofthe adsorbed polymer layer via re-conformation and/or degradation [5].

Although the surface charge of both AKD and cationic retention aids lpositive, the retention aids have a significant influence on retenti on behavior [3].Previous work conducted on the interactions between AKD and retention aids hasshown that the AKD surface charge has a profound effect on AKD retention on itsown, but s soon as a retenti on aid cPAM) is added, it is a less significant factor [6].Others agree that adding cationic polymer to AKD and fibers leads to a higherretention, which increases sizing [7]. One suggestion to explain this behavior is thatthe increase in AKD retenti on is due to an increase in fines retention, though thispoint has not been fully explored. Another theory assumes that at a neutral pH, AKDparticles become amphoteric from the adsorption of anionic materiaI, and in thismanner a cationic retenti on aid is able to bridge the negatively charged areas [7].

Typically, AKD is stabilized by cationic starch, since the fibers are negativelycharged. This enables the AKD to deposit on the fibers by electrostatic attraction. Onthe other hand, the use of cationic retenti on aids allows anionic AKD emulsions to beused as well. In sorne paper mills, AKD is stabilized with anionic polymer, andalthough the AKD and fibers naturally repel each other, the cationic retention aidpermits bridging between the two.

49


63/112

Anionic AKD emulsions are advantageous because they can be stored at a50 solids content, whereas the cationic emulsions are not stable above 15 or 20solids content. This is most likely due to the selection o stabilizing chemicals ineither case. This drastically reduces the cost associated with transporting and storingAKD at a mill. N evertheless, AKD and fibers are both negatively charged, so aprecise balance o polyelectrolytes/retention aids must be added to optimizeperformance. Conversely, cationic AKD has good retention over a broader range oretention-aid dosages. In this chapter, we discuss experiments performed with bothcationic and anionic emulsions.

Many researchers predict that cationic AKD emulsions will interact withcP AM, owing to the amphoteric nature o stabilizing agents in the emulsion. In thesecases, cationic starch and lignosulfonic acid are commonly used as stabilizingpolymers [8]. However, the explanation o this mechanism is vague. Furthermore, itwould not apply in our case, since only cationic starch is used as a stabilizer.

3 3 Materials and Methods3 3 1 Materials

F or trials with cationic AKD emulsions, we used the emulsions described inChapter 2. These AKD particles have a particle diameter o 0.8 f.lm and anelectrophoretic mobility o 1.33 [(f.lmls)/(V/cm)]. Anionic AKD emulsions wereproduced using carboxymethylcellulose (CMC) as the anionic stabilizer. Thischemical (received from Aldrich Chemicals) has a molecular weight o 700,000 Da.The same procedures were used for emulsification, using 0.15g CMC: 1 g AKD. The

50


64/112

anionic dispersions were monodisperse with an average diameter of 0.7 J.lm. AlIemulsions were diluted to a 3g/L solution, to extract precise amounts for testing smalIquantities. Tap water, with a pH of 6.4 and a conductivity of 280 J.lS/cm was used inall experiments.

We used Percol292 as the cationic polyacrylamide (CIBA, Inc.), which has adegree of substitution of 20 and a molecular weight of 3xl06 Da. Several otherpolyacrylamide polymers were tested, but since they all gave similar results, they arenot discussed here.

The fibers we used were bleached softwood kraft fibers (BSK) obtained fromDomtar. Fibers were dispersed and diluted to produce a 0.6 consistency pulp (thesame consistency used for the twin-wire sheet former).

3.3.2 Methods: Photometrie Dispersion Analyzer Tests PDA)PDA tests were carried out in a IL beaker with tap water, using the same

configuration as presented in Figure 2.4. One gram of AKD (in emulsion) was foundto produce enough turbidity such that accurate changes in flocculation could bedetected. The amount of cPAM added was between 0-10 g/g AKD. A typical amountused in industry is 1 mg cPAM/g fiber lg cPAM/g AKD). Stirring was constant at100 rpm while the dispersion was circulated at a flow rate of 150 mL/min. Cationicpolyacrylamide was added 30 seconds after the start of each mn. Since the refractiveindex ofAKD is lower than that of PCC, the apparatus needed to be adjusted to makeit more sensitive. The RMS Gain was increased to 5.3 to c1early detect changes in thestate of flocculation. The RATIO output was monitored for ten minutes.

51


65/112

The PDA apparatus was used to investigate the behavior of AKD with cPAM.Since both compounds are positively charged, we expected no interactions betweenthem, though it was necessary to verify. In addition, anionic AKD emulsions weretested to compare how a negatively charged emulsion might behave.

3 3 3 Methods: Adsorption KineticsAdditional experiments were performed to determine the kinetics of AKD

deposition on fibers in the presence of cPAM and under shear. A 0.6 wt fibersuspension was added to 500 mL of tap water. This consistency was chosen becauset was the concentration used in the twin-wire sheet former experiments, and was held

constant since fiber concentration can have an effect on flocculation.AKD was added to the fiber suspension being stirred. Fifteen seconds later, a

specified amount of cP AM was injected; this was designated as time zero. A 200-mesh screen (74 -lm openings) was used to extract a 2-mL sample free from fibersand fines. Samples were obtained at 15s, 30s, Imin, 3 min, 5min, and every 5 minutesthereafter. A UV-VIS Cary l spectrophotometer (Varian, Inc.) was calibrated at350nm; this wavelength was chosen for its good transmittance of AKD. A filtratefrom the fiber suspension prior to the addition of AKD was used in the reference cell.After AKD addition to fiber suspension, we measured the absorbance of the filtrate,from which we obtained the amount of AKD that was not retained, and thencalculated the amount of AKD that had deposited onto the fibers, as a function oftime and cP AM addition. A small experimental error occurred due to the slightlynegative readings obtained by the spectrophotometer at zero concentrations.

52


66/112

3 4 Results

3 4 1 KD Flocculation

Since we expected an anionic AKD emulsion (a-AKD) to be flocculated bycationic polyacrylamide, we tested this first. The flocculation that was detected withthe anionic emulsion was not detected with our original cationic emulsion and cP AM.This confirmed our initial premise that cationic AKD and cP AM do not interact.

Figure 3 1 shows vials containing g anionic AKD and different quantities ofcP AM. With the addition of cPAM, it is not until the 50-mg/g-addition point thatdestabilization occurs. At lower additions, such as those used in our experiments,anionic-AKD and cPAM flocculate to a much lesser degree. Large flocs, as seen invial 7, only form at high quantities of cP AM.

Figure 3.1: Photograph of vials with g a-AKD and various quantities ofcPAM;Amounts (0-50) indicate [mg cPAMIg anionic AKD].

53


67/112

3.4.2 Cationic AKD Deposition on FibersKinetic adsorption tests were performed with the cationic AKD emulsion,

fibers, and retention aid. An chemicals were added in quantities that approximateindustrial papermaking conditions. AKD addition was held constant at 10 mg/gfibers, while cPAM addition varied from 0-lOmg/g fibers. As shown in Figure 3.2below, in the absence of cPAM, AKD retention reaches approximate1y 75 , andbegins to slowly decrease to about 60 . When cP AM is added, AKD retenti onreaches 100 (lOmg AKD/g fibers).

10 . ~ ; ; ' - . ' ~ - ~ ~ - : ' : : ' : : : - ~ ' : J ~ ' ~ . : . - . = = = ~ t = = ....- -. ./ / / ................................./ / . . -v /8 : /. . /

. :l1li ../_.f . - III-.L.~ / . . . - - l 1 l i - . -E J/ : / -.-tIII.-.- 111 . .6 JJ 1 / If / .r

~ / /~ 4 /.ClCl~ 2

[mg cPAM/g fiber5]_ . l1li. _ . control-+ - 0.05mglg. . . . . 0 1 mglg0.25 mglg

- - 1'- 2 mglg- -+- 4 mglg5mglgO_ _ - - - . - - - - - . - - - ~ / ~ - - ~ = = ~ = = ~ = = ~o 200 400 600 900 1200 1500 1800

Time (5)

Figure 3.2: AKD deposition onto fibers, at 10 mg AKD/g fiber addition, andvarying cP AM addition. Inset: [mg cP MIg fibers]

At an concentrations of cP AM, the initial rate of flocculation is the same, as AKDdeposition reaches ~ 2 0 retention in the same time. At low cPAM concentrations(below 0.25 mg/g) , the time to achieve maximum adsorption is short (about threeminutes), whereas at high cP AM concentrations, the time required for the AKD to

54


68/112

reach 100 is prolonged, by up to 10 minutes for dosages above 4 mg/go The fourmain features of the figure above are described below.

1 Without cP AM, AKD deposition goes through a maximumIn the absence of a retention aid, cationic AKD partic1es are deposited on

negative fibers by heterocoagulation [8, 7]. The AKD partic1es, which are coated withcationic starch, adsorb to fibers through a weak bond causing sorne AKD partic1es todetach, leaving starch on the fibers (Figure 3.3).

+ AKD coated with cationicstarch adsarbs ta fibre surface Balance between attachment anddetachmentFigure 3.3: AKD deposition to fiber surface

After a maximum peak in AKD deposition, the AKD partic1es begin to detach.The transfer of starch from AKD to fibers explains why the deposition decreases aftera certain time.

2. cP M increases AKD deposition, despite the fact that cPAM does not adsorb onAKD coated by cationic starch

In the presence of cPAM, starch-coated AKD partic1es can adsorb either to thebare fiber or to the cP M previously adsorbed to the fiber (Figure 3.4 a,b).

55


69/112

Ca low [cP M]

AKD +) adsorbs to fibresurface

AKD detachment AKD heldmore strongly by cPAM

Cb high [cP M]

cPAM adsorbs to fibre surface

AKD adsorbs to cPAMFigure 3.4: AKD deposition on fibers a at low cPAM dosages; b) at highcP AM dosages

t may seem contradictory that AKD deposits on a cP AM molecule adsorbedto the fiber surface, especially since we found in the previous section that cationic

AKD particles and cPAM do not interact. However, this is possible by asymmetricpolymer bridging. The theory behind this phenomenon is presented below.

Heteroflocculation is not expected when a polymer is added to a mixture otwo types o particles and polymer adsorption occurs on only one type. A previousstudy by van de Ven et al. proposed a mechanism o asymmetric polymer bridging[9]. This theory states that heteroflocculation can occur even for systems in which thepolymer does not adsorb on one o the two flocculating species, provided that thepolymer upon adsorption on Particle 1 acquires the ability to adsorb on Particle 2(Figure 3.5). We find that cPAM in free solution does not adsorb to AKD, but uponadsorption to fibers, cPAM can adsorb onto AKD.

56


70/112

A +c

no dsorption

A + :: : :......... .:-;:::.:.:< ::::::: : - - - ' ~ ~

A: cPAM B:AKD C: fiberFigure 3.5: Asymmetric Polymer Bridging Mechanism

Whether a freely dissolved polymer adsorbs onto a particle depends on thecompetition between a gain in enthalpy and a loss of entropy upon adsorption.Adsorbed macromolecules have fewer configurations, so the loss in entropy is lessupon adsorption onto another surface. This permits a mole cule to bridge two particleseven when it is adsorbing onto only one of them. f the polymer adsorbs onto onlyone surface, one would expect that steric stability is not possible and, as a result,heteroflocculation should occur at aH polymer concentrations. This phenomenon hasbeen observed with polyethylene oxide PEO) and clay with fibers [9], and here withAKD and cP AM with fibers.

Also puzzling is why the AKD-cPAM-fiber bonds would be stronger than the

AKD-fiber bonds. InitiaHy, the starch-coated AKD particle adsorbs to the cPAMalready adsorbed on the fiber. One explanation for the increased strength involves thereconfiguration of starch and its displacement from the gap. f the starch were tospread, as illustrated in Figure 3.6, AKD would be in direct contact with the cPAM,most likely producing a stronger bond. This spreading not only increases the bond

7


71/112

strength, but secures the AKD partic1e so it is less likely to detach. In addition, AKDis still able to quickly adsorb to fibers independently of cP AM. In solutions with lowcP M concentrations, a combination of these weak and stronger bonds exists, asshown in Figure 3.4a.

(a) (b)

starch

cP M fiberWeak bond: cPAM has little Strong bond: cPAM is likely to haveaffinity for cationic starch stronger affinity for AKD than for starch

Sorne starch rnay transfer to fiber)Figure 3.6: Increased Bond Strength by Starch Re-conformation( a) Initial adsorption of starch to cP M(b) Spreading of starch allowing for direct AKD adsorption to cP M

3. The initial kinetics (up to 20 ) is nearly independent of cP M dosageAKD deposits on bare fibers and cP AM-coated fibers at the same rate,

suggesting that in both cases there is no (or a negligible) energy barrier in the AKD-fiber interaction that might slow deposition. At first glance, this was surprising since

AKD does not adsorb on cP AM, but was explained by asymmetric polymer bridgingin the previous section.

Equation 3.1 describes the kinetics for regular homoflocculation. n optimumin flocculation efficiency a) is reached when the fractional surface area covered (8)is K

58


72/112

=2B I-B)For heteroflocculation, the flocculation efficiency equals:

a = I 1 B2 +B2 1 I )

(3.1)

(3.2)In asymmetric polymer bridging, we find the following relationship [9]:

(3.3)slllce 92 = O When complete fiber coverage B=I) is achieved, the maximumadsorption efficiency a) is reached, indicating the maximum rate of flocculation.Any dosage corresponding to full coverage or more will yield the same rate a Wewould expect high concentrations of cP AM to produce identical responses for initialflocculation, as seen in Figure 3.2. Since there is no energy barrier, the initial rate ataIl cP AM dosages, as weIl as for AKD depositing on bare fiber patches is the same.As more time elapses, the curves digress due to other processes, such as polymerrearrangement, which is discussed below.

4. With xcess cPAM, the reaching ofmaximum retenti on is delayedWhen a high concentration of cP AM is added to the AKD-fiber suspension,

the fiber surface is coated exclusively by cPAM molecules (Figure 3.4b). Thesluggish process of attaining 100 AKD retenti on occurs due to re-conformation anda slower, but stronger process of AKD adsorption to cP AM. One possible scenario i

Documents

Alkyl Ketene Dimer and Precipitated Calcium Carbonate Interactions in Wetend Papermaking