comparing papermaking wet-end charge-measuring techniques

142 November 1995 Tappi Journal

Charge Measurement

It is now well recognized that mea-suring papermaking furnish

charge is an important if not critical

contribution to balancing wet-endchemistry (1–4). Full optimizationof chemical additive efficiency can-

not be realized except where wet-end colloidal forces have been ap-propriately adjusted to worktogether with (generally fixed) me-chanical or hydrodynamic forces toachieve an ideal sheet structure (5).Fortunately, a variety of charge mea-surement techniques are now avail-able. Together they possess thecapabilities for assessing most as-pects of colloidal chemistry that areimportant to papermakers. How-ever, the application strategies forthese techniques currently used inthe paper industry have often beendeveloped without sufficient under-standing of their interrelationships,sensitivities, and interferences (6).The purpose of this work was to im-prove our knowledge of the abilityof various wet-end charge techniquesto sense papermaking furnish varia-tions that affect wet-end chemicalbalance. We also sought to catalogthis sensitivity/interference informa-tion to be able to provide recommen-dations to papermakers that bestmatch charge techniques and appli-cation strategies to specific paper-making problems.

Our approach to this study ofcharge techniques and applicationsis to make comparisons with respectto a common “benchmark” tech-nique, automated electrophoresis(7). Automated electrophoresis bythe principle of Doppler electro-phoretic light scattering (DELS) hasbecome a good benchmark (8) forcharge distribution analysis of com-

Comparing papermaking wet-endcharge-measuring techniques in kraftand groundwood systemsNigel D. Sanders and John H. Schaefer

ABSTRACT: Control of the papermaking process to maximizeefficiency of chemical additives relies on careful balancing ofcolloidal and hydrodynamic forces. Charge characterizationmethods for papermaking furnish colloids have becomeincreasingly important as the paper industry moves towardgreater use of mineral fillers and recycled fiber. In this paper, wecompare a high-precision approach to colloid chargemeasurement, Doppler electrophoretic light scattering (DELS),with methods based on pad streaming potential (SP), andcolloid titration with standard polyelectrolytes (CT) to acolorimetric or streaming current (SC) endpoint. Simplelaboratory synthetic furnishes consisting of kraft or groundwoodpulp and precipitated calcium carbonate (PCC) can be used togauge the relative sensitivity of the various methods to thecharge-modifying effects of suspension ionic conditions andpolyelectrolyte paper chemical additive levels. The discussionfocuses on the roles of particle size and charge heterogeneity indetermining each method’s sensitivities and precision.

KEYWORDS: Calcium carbonate, colloids, colorimetry, elec-tric potential, electrophoresis, furnish, ground wood, hydrody-namics, kraft pulps, measurement, polyelectrolytes, surfaceproperties, volumetry, wet ends, zeta potential.

Sanders is a project manager and Schaefer is a research investigator at SpecialtyMinerals, Inc., 9 Highland Ave., Bethlehem, PA 18017.

Charge Measurement

Vol. 78, No. 11 Tappi Journal 143

plex fine-particle suspension mix-tures (9–11). The data from theDELS technique are expressed interms of particle zeta potential (ZP).Each comparison is presented as a“one-on-one” case study, since thefurnish systems used are different.

There are two wet-end chargemeasurement techniques consideredfor comparison to DELS. The first iscolloidal titration (CT), in which thecharge of a suspension of solubles

and fine particles, and in some casesfibers, is determined by measuringthe amounts of standard polyelec-trolytes needed to achieve an “iso-electric” endpoint (12); this endpointis detected by colorimetry (13) orstreaming current (14). The secondmeasurement technique consideredis streaming potential (SP), in whichthe charge of a fiber pad is deter-mined in the presence of the flow of a

solubles/fines-containing suspension(15, 16).

Halabisky (12) compared ZP anda ratio of CT values (“CTR”) for an-ionic and cationic polyelectrolytestandards as a means to control alumand cationic starch usage in mill stud-ies. He found CTR to be a more sen-sitive method than ZP for thispurpose, although the two methodsagreed on the region of charge neu-trality in most cases. Penniman (15)found some differences in the re-sponse of ZP and SP techniques tocationic polyelectrolyte addition, es-pecially with regard to apparent tran-sients in the SP signal that varied inmagnitude and relaxation time withpolymer level and pretreatment.Scott et al. (4, 17) confirmed the SPtransient phenomenon of Pennimanand showed that it often could beseen in ZP testing as well. Scott et al.(4) concluded that SP and ZP mea-surements generally correlated wellin laboratory testing of chemical andmechanical pulps with various chemi-cal additives. Scott and Koethe (17)have explained the transients interms of the polymer reputationtheory of Lindström et al. (18), whichpostulates a rapid initial adsorptionof the additive on the surface of thefibers (or fines) followed byreconformation and/or diffusion intothe fiber wall.

A previous study (19), which com-pared CT (colorimetric endpoint)with ZP, concluded that, with simple,single-component suspensions of pre-cipitated calcium carbonate (PCC),the two techniques correlated quitewell, while the addition of pulp to thePCC immediately produced complex,addition-order dependent effects thatthe two methods sensed differently.Mill data on a starch-loaded fine pa-per grade showed a highly nonlinearZP/CT correlation, with the furnishapproaching neutrality first in ZPand then in CT; this was consistentwith the pulp/PCC/starch laboratorymodel system and earlier work byBrouwer (20).

1. Comparison of zeta potential with colloid titration and first-pass retention (headbox)

2. The effect of first-pass retention on headbox zeta potential distribution

70�

60�

50�

40�

30�

20�

100 2 4 6 8 10

MEAN ZETA POTENTIAL, -mV

TO

TA

L F

PR

, %

CH

AR

GE

DE

NS

ITY

, (µe

q/L

)/10

12 14 16 18 20

FPR Charge density

ZETA POTENTIAL, mV

SC

AT

TE

RIN

G IN

TE

NS

ITY

0 -40 -30 -20 -10 0 10

30% FPR 60% FPR


Charge Measurementcase). The high-molecular-weightAPAM transient relaxation ratedepended on the level added andthe type of cationic preparationused (alum vs. polyamine). For-mation of calcium lignosulfonatesoaps may support foam, whichcan render on-line units inoper-able.

Case Study IV: Streamingpotential (groundwood pulpsystem)Furnish system: Laboratory study.100% groundwood pulp [softwoodbleached chemithermomechanicalpulp (BCTMP), 200-mL CSF, 80 ISObrightness]; 0–40% precipitated cal-cium carbonate (PCC) filler(scalenohedral, 1.4 µm); alum: 0–0.45%; cationic polyamine: 0–0.325%;anionic polyacrylamide: 0–0.1%; cat-ionic polyacrylamide (CPAM): 0–0.35%; total solids were 0.5–0.7%;conductivity was 0.1–1.7 mS/cm; pHwas 3.9–10.0; and calcium hardnesswas 0.5–56.4 ppm (as Ca).

Results. The results of six runstesting the sequential effects of sys-tem variables previously listed aresummarized below with the specifictesting sequence given first.

Run 6: Groundwood pulp, salt,PCC, pH, alum, APAM. Excess airin the SP system produced falsereadings during the first 80 min ofthe run (see Fig. 8). ZP was initiallymuch more anionic then SP. Unlikethe kraft pulp systems, the increasedconductivity pushed the ZP morecationic (–43 to –21 mV) and nar-rowed the distribution by 50%, whilethe addition of PCC rebroadened thepeak width and pushed the ZP an-ionic. Increasing Ca2+ by loweringpH with HCl caused a sharp ZP cat-ionic swing (–27 to –15.5 mV), whichgreatly improved the agreementwith SP while the distribution againnarrowed by about 50%. (See Fig.9.) The second alum addition causeda narrow but sharp SP cationic peak,while the APAM contributed to theanionic transient, which decayedfaster than in the kraft system.

Run 7: Groundwood pulp, salt,PCC, pH, cationic donor, APAM.In sharp contrast to the kraft CaseStudy IV, the ZP and SP pulpbaseline agreement was poor (–34.5mV vs. –6.3 mV). (See Fig. 10.) In-terestingly, groundwood SP is lessthan that of kraft, while ZP is higher.Once again, the ZP went more nega-tive after the 20% PCC addition andthen moved 4 mV more cationic af-ter the 40% PCC level increase. ZPand SP agreement greatly improvedafter the pH was lowered with HCl.The addition of cationic polymer isreflected in the ZP change, with littleor no change to SP. Both APAMadditions produced similar tran-sients to those seen in kraft pulpexperiments.

Run 8: Groundwood pulp, pH.This pH experiment in an unfilledgroundwood system showed the ZPand SP pulp baseline agreement waspoor (–24.5 mV vs. –4.4 mV). ZP fol-lowed pH trends similar to the kraftpulp while SP did not. ZP distribu-tion width increased during high-pH,low-calcium periods. Visual alkalinedarkening and calcium ion concen-tration were reversible when mov-ing between high and low pH.

Runs 9, 10, and 11: Filled ground-wood furnish, APAM, CPAM, andcationic donor levels. ZP and SPpulp baseline agreement was againpoor. The ZP moved in the positivedirection as the conductivity in-creased with the addition of KCl,and both ZP and SP went negativewhen PCC was added. Agreementbetween ZP and SP improvedgreatly with the addition of HCl,which lowers the pH and increasesthe calcium ion concentration. ZPand SP follow a linear trend with theanionic polyacrylamide (APAM) andcationic polyacrylamide (CPAM)level additions (see Fig. 11) with SPtransients observed. ZP and SP fol-low a linear trend with the CPAMlevel additions with SP transientsobserved. ZP follows a linear trendwith the cationic polyamine level ad-ditions, and there are no transientsas seen in the kraft pulp experi-

ments. SP does not change with CPAaddition in this groundwood system.

Summary.

• Unfilled groundwood furnish.Unlike the kraft pulp experi-ments, the starting values of ZPand SP were both in poor agree-ment, with an average of 29.8 mVdifference for the six groundwoodexperiments. Increasing the con-ductivity with KCl and loweringthe pH to 4.0 caused the ZP dis-tribution to narrow, and the aver-age value moved more cationic;the SP showed little effect of thesevariables. Visual alkaline darken-ing and calcium ion concentrationwere reversible when moving be-tween high and low pH.

• PCC-filled groundwood fur-nish. Adding PCC to the ground-wood pulp system broadened theZP distribution and pushed thevalues an average of 5.1 mV moreanionic, while the SP negativeshift was only half of that value.Increasing the dissolved calciumion concentration by an averageof 33.6 ppm as Ca (by loweringthe pH an average of 1.4 unitswith HCl), the ZP distributionnarrowed, and the values movedan average of 9.4 mV more cat-ionic with little effect to SP. Theaddition of acid, alum, and cat-ionic donors failed to produce theSP transients seen during thekraft fiber experiments (CaseStudy III), while high-molecular-weight APAM and CPAM causedSP transients with a shorter de-cay time than those with kraftfibers.

Discussion—furnishes

Soluble charge vs. fines/fillerWe have found that Ca2+ and cat-ionic starch affects fine-particlecharge more than soluble chargewhile cationic polymers do the oppo-site. Soluble anionics from pulps arenot well neutralized by starch, butPCC can be highly effective at col-

Vol. 78, No. 11 Tappi Journal 149

lecting this “trash” if added to thesystem early. Anionic “trash” col-lected by PCC charges furnish ZPbut may not be titratable.

When ZP is compared to thecharge demands of furnish fractionsfrom a typical alkaline system, thick-stock fines CDs correlate fairly wellwith ZP data (on fines), whilesolubles are significantly more cat-ionic and fibers are more anionic(have a lower and higher cationicdemand, respectively). In contrast,the more complex colloid mixture inthe thin-stock loop shows no corre-lation of ZP with CT data on any sizefraction. ZP is consistently more cat-ionic than the solubles at theheadbox. The surface characteristicsof the thin-stock fines have been suf-ficiently altered by high-molecular-weight polymer layers to negate anysimple relationship between particle

charge (ZP) and polyelectrolyte ad-sorption.

Fines/filler vs. fiberKraft pulp fines have similar sur-face chemistry to fibers at equilib-rium conditions. Fiber chargedynamics are strongly influenced byconductivity, and fiber surface chem-istry “shock” can be much largerthan that for fines/filler particles.The magnitude and relaxation timeof SP transients depend on polyionmolecular weight and fiber precon-ditioning. Narrower ZP distributionsare often associated with narrower(or zero) transients in SP.

Comparison of SP and ZP withBCTMP pulp shows that, in contrastto kraft, groundwood fibers and fineshave a different charge behavior (fi-bers lower charge, fines muchhigher) except in systems with lowpH or high Ca2+, where fines ZP is

reduced. Polyelectrolyte additive ef-fects on groundwood/PCC mixtureSP are generally smaller than withkraft, especially with lower-molecu-lar-weight cationics, which barelychange SP. These studies may im-ply a more hydrophobic nature ofgroundwood fibers compared togroundwood fines and kraft pulp.High-molecular-weight polyelectro-lytes give transients in SP forgroundwood fibers with shorter re-laxation times than kraft fibers.

Discussion—techniques

PrecisionPrecision frequently depends moststrongly on the level of charge het-erogeneity. In this study, precisionwas adequate to detect the smallvariations necessary with all tech-niques evaluated.

SensitivityThe relative sensitivities of the threetechniques for the variables testedin this study are listed in Table I.

Conclusions

The choice of an appropriate strat-egy for applying wet-end chargemeasurement techniques to a givenpapermaking furnish system relieson knowing the sensitivities of thetechniques for the variables of inter-est. In this paper, we have providedcomparative information between abenchmark technique, zeta potential(ZP) by Doppler electrophoreticlight scattering (DELS), and twoother common techniques, colloid ti-tration (CT) and streaming poten-tial (SP), which are generallysensitive to different aspects of thefurnish. We found that there are in-deed distinctions among the tech-niques when they are tested bytypical alkaline papermaking vari-ables. The benchmark technique,DELS, has the highest sensitivityfor fines/filler particles and the ef-fects of changes in alum and cationic

11. Effect of additive level on zeta potential vs. streaming potential

ADDITIVE LEVEL, %

ZE

TA

PO

TE

NT

IAL

, mV

0�

-5�

-10�

-15�

-200 0.05 0.10 0.15 0.20 0.25 0.30 0.35 0.40

ZP APAM ZP CPAM ZP CPA

SP APAM SP CPAM SP CPA

Zeta Streaming Colloidalpotential potential titration

Fines Fiber Pulp solublesFillers PAMs Cationic donorsAlum - -Cat. starch - -

I. Relative sensitivities of three charge techniques to furnish variables


Charge Measurementstarch. Streaming potential is bestused when the important variablesare fiber charge and retention aidlevels. Colloid titration has advan-tages when sensitivity to anionicsoluble “trash” and cationic promot-ers is needed. In general, correla-tion between DELS ZP and SP isquite good, while ZP and CT corre-late poorly, if at all. We must there-fore view ZP and CT ascomplementary techniques (6, 19),while ZP and SP are comparative(4).

The application strategy choicealso involves considering the pur-pose of the testing regimen. Estab-lishing a baseline response surfacegenerally requires off-line wet-endprofiling, which may involve two orthree different methods (ZP and CT,for example). Exceptional trends orcritical sensitivities may justify com-ponent separation techniques. Oncethe baseline responses have beendefined, single-point control moni-toring can begin at one or morepoints that were indicative of thehighest sensitivities. For this pur-pose, on-line sensors are ideal, andSP types, which have the necessarysensitivity and precision based onthis and other laboratory studies, arenow available (4, 15, 16). Develop-ment of filtration algorithms for theon-line sensor’s response will then

allow the papermaker to distinguishin- and out-of-spec wet-end chemi-cal balance. Out-of-spec flags willtrigger direct actions to resolve afamiliar imbalance or off-line sys-tem profile measurements to inves-tigate an unusual circumstance.

The ultimate goal for wet-endmeasurements is to have the knowl-edge available to respond to systemvariables in a proactive fashion asopposed to waiting for the onset ofsignificant production loss. We be-lieve that modern wet-end chargetechniques are making headway inachieving this important total qual-ity goal. TJ

Literature cited

1. Strazdins, E., Tappi 48: 157(1965).2. Stratton, R. A. and Swanson, J. W., Tappi

64(1): 79(1981).3. Lindström, T., Electrokinetics of the Pa-

permaking Industry, Paper Chemistry,Chapman and Hall, New York, 1991, pp.25–42.

4. Scott, W. E., Hand, V., Koethe, J., et al.,TAPPI 1993 Papermakers ConferenceProceedings, TAPPI PRESS, Atlanta, p.591.

5. Sanders, N. D. and Bashey, A. R., TAPPI1991 International Paper Physics Con-ference Proceedings, TAPPI PRESS, At-lanta, p. 473.

6. Patton, P. A. and Lee, D. T., TAPPI 1993Papermakers Conference Proceedings,TAPPI PRESS, Atlanta, p. 554.

7. Sugrue, S., Oja, T., and Bott, S., Ameri-can Laboratory 21(2): 98(1989).

8. Camp, R., Benchmarking: The Search forIndustry Best Practices that Lead to Su-

perior Performance, ASQC Quality Press,Milwaukee, 1989.

9. Sanders, N. D. and Schaefer, J. H.,TAPPI 1989 Papermakers ConferenceProceedings, TAPPI PRESS, Atlanta, p.69.

10. Sanders, N. D., Proceedings of the Engi-neering Foundation Conference on Dis-persion and Aggregation: Fundamentalsand Applications, Palm Coast, FL, March1992, p. 183.


12. Halabisky, D. D., Tappi 60: 125(1977).13. Ueno, K. and Kina, K. J., Chemical Edu-

cation 62: 627(1985).14. Bley, L., Paper Technology 33(4):

32(1992).15. Penniman, J. G., Tappi J. 75(8): 111(1992).16. Jazayeri, S., Proceedings of PIRA Con-

ference on the Chemistry of Papermak-ing, PIRA International, Leatherhead,Surrey, UK, 1993, Paper 19.

17. Scott, W. E. and Koethe, J., TAPPI 1993Papermakers Conference Proceedings,TAPPI PRESS, Atlanta, p. 569.

18. Lindström, T., Winter, L., Wågberg, L.,et al., Journal of Colloid and InterfaceScience III(2): 537(1986).


20. Brouwer, P. H., Tappi J. 74(1): 170(1991).

We would like to thank William Scott of MiamiUniversity, Oxford, OH, for the use of his equip-ment and technical assistance, and Bruce Evansand the members of the Specialty MineralsGroundwood team.

Received for review Jan. 7, 1995.

Accepted April 10, 1995.

Presented at the TAPPI 1994 PapermakersConference.

comparing papermaking wet-end charge-measuring techniques

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