Rheological behaviour of polymer flocculated latex suspensions

Colioids and Surfaces, 7 (1983) l-13 Eketier Science Publishers B.V. , Amsterdam

1

-

Printed in The

Netherlands

RHEOLOGICAL BEHAVIOUR OF POLYMER FLOCCULATED LATEX SUSPENSIONS ROBERT

J. HUNTER,

School of Chemistry, (Accepted

ROBYN

Uniuerrity

in final form

MATARESE of Sydney.

1 October

and DONALD Sydney,

H. NAPPER

N.S. W. 2006 (AustmtiuJ

1982)

ARSTRACT Polymer floccul-rted latex suspensions exhibit rheological characteristics which ate similar to thosa of s&-coagulated latex suspensions: plastic-pseudoplactic behaviour with a well defined Bingham yield value, ra, and a constant plastic vir=sitY, ~PL. at shear ratea larger than a certain critical value, ts,. In aall+zoagutated aydtcms the elastic fly model can be used to develop correlations bctwecn the flow propertfcd (w) qpt and 0.) and the principal collold charactarS.stfcs; in particular the eleclrok3nctic (r-) potential which determiner the magnitude of the attractive force between the partides. Thts paper exemhms the extent to which the elastic floe m&e1 can be used to understand, at the microscopic level, the dominant prom occurring in a flowh~g p6Jymer-flocculated latex suqendon. At pH 10 when the (cationic) polymer is unchwed the flow parameters vary in a regular manner with s and the claalic floe model olfera some InsIght into the mScroscoplc behaviout, provided one recognism that polymer adsorption alters the effecfive volume fraction of the solid. At pM 4 tho highly positive polymer interacts electrostatically with the negative latex particles and the flow behaviour suggests that the partictc-particle interactions are electrostatic in nature. There is some direct evidence of polymer bridging between wrticlcs, in the “stringy” texture of the floca, but only the gently stirred systems at very high polymer It?vcJs show flow characteristtcs which might be interprefd by polymer bridging. The present work involved subjecting the system to high shear rates (> 1000 S-I) before measurement and must be supplemented by work on systems with less vi&M shear history wham polymer bridging might be more 6ignfficanL

INTRODUCTION

The rheological bchaviour of salt-coagulated latex suspensions is weli de“‘elastic floe” model of Firth and Hunter 11). The present work is an attempt to determine the extent to which the concepts embodied in that model can be extended to cover the behaviour of polymer-flocculated scribed by ttie

systems.

Both saltcoagulated and polymer-flocculated systems exhibit the same general flow properties - non-Newtonian (plastic-pseudoplastic) Row with a well defined Ringham yield value (7~) and a constant plastic viscosity (?p,,)

0166.6622/83/$03.00

Q 1983

Elsevicr Sclencc Publishers

B.V.

2

20’

15.

z dylW3 cm*

10.

_ Fig-l. Typical basic flow diagram for a polymer flocculated latex system. The plot shown is for pil 4 wilh % p/i = 0.6, TB’ 2.98 I 0.72 dyne cm-‘; D, = 180 s-‘;qpt = 2.84 f 0.35 CF.

at high shear rate, as indicated in Fig.1. The critical shear rate, DO, at which the r-D curve becomes linear is also an imptiant feature of the “‘elastic floe” model. Equations have been derived [I, 21 to relate these three quantities b the impwtant colloid chemical proprtics of the suspension (viz., the particle volume fraction, +p, the particlc radius, r, and the interparticle forces, as measured by the olectrokinetic (t-) potential.) For the present ~xwposcs, the most important feature of these equations is that all three flow quantities (7 8, Da and qpL) are directly determined by the maximum force of attraction between the ~xuticles, F M , as given by the DLVO theory of colloid stability [ 31: = r Il=d,

A 12 d;

(1)

where VT is the total @enCal energy of interaclion, A is the Hamaker constant and RI is the value of the interparticle distance, 11, which makes the force a maximum. The second term in parenthesa measures the double layer repulsion and for constant potential surfaces: fi

(d,) =

21TEK 1 + cxp (Kdl)

ia

where E is the pcrmittitity and K is the Debye-Huckel parameter. This expression (given in rational&d form [ 4) ) assumes that the double layer potential (in bhis case taken as 5) is small, which is reasonable for coagulated systems. The value of DO is given by:

3

where nF is the number of floe-floe bonds and qr is t.he viscosity of the suspension medium. The plastic viscosity is used to evaluate the quantity Cr, = #r/* which measures the degree of openness of the floe (Le., the amount of trapped solvent). tip is the volume fraction of fiocs which is determined from an expression suggested by Mooney [5i] : VPL

= qr

2.5#F

exp

l-k

‘Qp

(4)

where k ’ is here taken as 1.4, so that rll , becomes infinitely large as eF approaches the close-pack value. The elastic floe model suggests that CFp is given by [6]: CFP

-- F&l

=1.6+

b rls r2

(5)

b is a constant. TB iS giVHl by [2]:

where

r’pa*-

Da2 6 r3

lr1s* #P~-‘&P

where a is the floe radius and d is the particle-particle bond stretching distance. The flow propertics are thus strongly dependent on the c-potential (through Eq. (1)) and the relation between them and g can be used to evaluate d,. When a cationic polymer is used to flocculate a negatively charged latex suspension it is obvious that ehxtrostat-ic interactions will be important. WC would hope that a detailed study of the flow bchaviour of such a system might make it possible to distinguish between situations involving purely electrostatic effects and those involving polymer bridging between the particles. It must be pointed out, however, that the elastic floe modeX was developed to describe systems which had first been subjected to a rather high shear rate (- 1600 8-l). This had the effect of establishing fairly well defined flow units and improving the reproducibility of the flow behaviour. Such a pretreatment procedure when applied to a polymer-floccuhrtcd system will certainly have a profound effect on the floe structure and will almost certRinly reduce the significance of the bridging processes which occur under milder pretreatment processes. EXYERIMENTAL

illa teriats (a) Latex: The poly(methy1 methacrylate) (PMMA) latex was prepared by emutsion polymerization as described by Kotera et al. [I] and extensively

4

dialysed against distiiled water before use. The particle radihs, determined by ultracentrifugation, was 135 f 5 nm. The $-potential was determined at an ionic strength of lO_” iIf (in KCI) at pH 4 and 10 by microelectrophoresis (Rank Mark II Apparatus). (b) Cationfc Polymer: This was a copolymer of acrylamide (75% w/w) and dimcthylaminorthyl methacrylate (DMAEM) of average mofecutar lveight 5 X 10" and an intrinsic viscosity of 9.6 dl/g (supplied by Catoleum P/L, Botany, NSW). Its charge status at pH 4 and IO was determined by potcntiomctric titration. It is essentially uncharged at pII 10 and is fully charged at pIi values below about 5,

All rhcological experiments were performed at a final part.icle volume fraction (Q*) of 0.063 and a constant electrolyte concentration of IO-’ nf KCI. Flocculation was studied at pH 4 and pH 10. Individual systems wxc charautctiscd by pII ancl polymer: particle ratio, express& as a percentage (%Jp/l): ‘;fup/l=

/w/lOLQppp

(71

whcrc p = volume of added polymer solution (ml); c = concentration of polymer solution (mg/ml): I, = volume of latex suspension (ml): pp = density of latex particles (g ml-‘). The values of % p/1 used wcrc: 0.35,O.G. 0.8. 1.0, 1.5 and 2% for the strongly stirred systems and similar values for the gcnlly stirred systems. Polymer solutions of different concentrations were prepared so that the final solution could be made up in each case by adding polymer to latex suspension in the same ratio, after separakly adjusting the pli of the! solutions. KC1 was added to the latex in sufficient concentration to yield t-he desired ionic strength in the final solution. The polymer solution was added slowly from a pipette to the magnel;ically stirred latex suspension and stirring continued for a further S min. Although the shear rate in this process is variable and unknown it would not be expected to cxcecd about 60 s-l and is, hence, considerably smaller than the value imposed before measurement-s of the rheological characteristics are made. Nevertheless, the actual mixing speed at this stage In thr! procedure turns out to be of critical importaIlcc (see below), Hocculafion efficiency. This was measured by determining the sedimentation volume both under gravity (settling for 3 days in 10 ml graduated cylinders) and by ccntrifugation {settling for 10 min at 300 rpm in 15 ml centrifuge tubes). The composilion of the supematant and the centrifugatc

were also determined as functions of % p/l at pH 4 and 10. Hilcotogkai behaviour. The shear stress-shear rate relation was dctermined for eacn sample in a Couette viscometer of small gap width (0.3 mm) over the range from O-1300 s-’ at 25OC beginniug at the high shear end of the range, as previously described [ 11.

Electmkinetic (P-J potential measurement. Values of C were calculated from the measured electrophoretic mobility using the calculations of Loeb et al. [S] for KU = 14. In order to ensure that the measured f-potential was relevant to the rheological measurement, each system was cS.kki into two sub-samples. One sub-sample was examined rheologically and then a small drop of that sample was added to the supcmatant obtained after centrifugation of the second sub-sample. Adsorption of polymer by latex. Adsorption studies were conducted in acid (pH 3) and basic (pH 10) solution using the same mixing and stirring procedures as described above for the rheological measurements. The solution polymer concentration before and after adsorption was determined by separating off the latex by centrifugation and titrating the clear polymer solution with a negatively charged polymer (potassium poly(viny1 sulfate)) using toluidine blue (0.1% w/w aqueous solution) as indicator. The centrifugation was done in two steps: first at 300 rpm for 5 min to remove flow and then at 36,000 rpm for 60 min to 1 h to remove individual latex particles; the second step was done in acid solut.ion (pH 3) to reduce the Edtent of hydrolysis of the PMAEM which occurs at high pII_ RESULTS I, Elccfrokinetic

(g-1 potential

The effect of positively charged polymer on the {-potential of the latex particks is shown in Fig.2, Reversal of charge occurs at pH 4 at a polymer particle ratio of 0.8% for systems which were stirred strongly during polymer addition and somewhat earlier with more gently stirred systems. The plateau at high polymer concentration is rather more pronounced in the latter case but otherwise Dhe behaviour is very similar in the two cases. At pH 10 the (uncharged) polymer reduces Zlre absolute magnitude of the c-wtential to about a quarter of the initial value.

0

10 .,,it,',/P&CLE R:.:IO(%l

Fig.2.Eledrokinetic stirring (U stirring.

potential 10 s-1); -0.: pH

a~ a function of polymer/particleratio. 0: PH 4 with weair 4 with strong stirring (D - 50 s-‘); -a -: pH 10 with strong

6

2. Plastic cismsity vahes of the plastic viscosity at pH 4 and 10 are showtl in Fig.3, as fun&ions of % p/l, There is a steady increase in qpL with increasing polymer additions st pH 10. At pH 4 the Wtions in qpL with added polymer are shnilar for the strongly stirred and the more gently stirred SyStiMS. and the correspondence is even more marked if one takes account of the fact that the point where f = 0 is slightly different in the two cases.

Fig.3. Plasticviscosity (qp~) as function of poSymer/particle The arrows indicate where f = 0 for pH 4. Fig.4. Bingham yield value (re) M functbn Fig.2. Arrows indicate where $ = 0.

3. Birlghatn yield

ratio. Symbab

of polymorlparticle

as for Fig.=.

ratio. Symbob 8s for

value

The relation between 78 and % p/1 is shown in Fig.4 for pH 4 and 10. At pH 10 there ia very little effect until the polymer/parbicb ratio exceeds lS_ By cuntrast, at pH 4 there is a significant effect at very low additions of polymer, especially in the systems which were gently stirred during polymer addition. Again the general features of the behaviour are qualitatively similar for the two different stirring rates, especially if the slight difference in gpotential bahaviour is taken irtr, account. In both Figs.3 and 4, the maximum in the rheoiogical parameter at pH 4 corresponds closely with the minimum in the 5 potential (c-f., Eq_ (1)). 4. Critical shmr

rate

Figure 5 shows the depe2dcnce of DO on added polymer. At pH 10 the value rises steadily over thp whole range. At pH 4 t.here is again a qualitative similarity between the runs made under strong and gentle stirring condi-

7

tions at small polymer additions although the maximum in DOno longer corresponds to f = 0 in the strongly stirred system. There is, however, a marked difference in the behaviour at large polymer additions with much higher values of DObeing exhibited in the gently stirred systems.

1

OlY

I

05

POLYHER/PAR~ICLE

I-5

RAtlO 1-A)

0

10

IO 05 POLw.kER/PARtICtE

Fig& Critical &ear rate (D,) as function of polymer/particle .\rrows indicate where f * 0. Pig.6 Gedimcntation parti& ratio. &did6 -0’; at pH 10: -I-_

vo!ume s;rd supcrnatant ConteZht

of

mJpt!m6tarIt

at

15 RATIO t-/m1

ratio. Symbols

as for Fig.2.

solids content as function of polymer/ pH 4: q ; sedimentation volume at pfj 4:

6. Fhccululion efficiency Figure G shows that whether judged on the criterion of settling volume or clarity of the supernatant, the optimum polymer concentration for flocculation is about 1% of the partlcle concentration. In fact, the polymer is of limited value at pH 10 since although some sedimentation can be induced at 1% polymer addition, the supernatant concentration remains high (at 1.02 wt% solids which is about 16% of the original material); at all other concentrations of added polymer there is no visible settling under gravity at pH 10. Microscopic observations of the aggregates at these two pHs reveal significant differences in their appearance. At high pH, the floes are more globular and compact but a signi5cant amount of the solid material is not involved in the floes. At pH 4 the floes are more ftlamentous, with all or almost all of the solid contained in the filaments. Although these ffocs settle slowly and trap a considerable volume of supernatant liquid, it would still be possible to effect a useful separation by drainage of the sediment on a filter pad. This is not true of the system at pH 10 where the filtrate would contain a significant fraction of the original solid.

8

6. Adsorpliun shrdies At. pH 10 essentially all of the added Imolyrnm was adsorkl by the latex pnrticlcs up to the maximum polymer addition. At pH 3 about 85--W% of the added polymer was admrbd at. all concentrations. At first sight it may appear stmnge that the dqpc of adsorption of the poeit.ivcly charged polymer is slightly less than that of the neutral enc. It should bc noted, however, lhat at pl,li 3 the polymer chain is l~ighly extendti? and in that. condition it is many Oimcs longer than tht! diameter of a latex particle. There arc then, llrcsumably , stcric constraints on t hr! ndsorpt.ion ~WOCCss.

The mechanism of polymer flocculation is discussed in dotnil by La Mer :~nrl lbaly IO], irascd on the model of Smollie and Ia Mer [lOI as modified by Healy nnd 1;~ Mcr [ 111 . ‘l’huir description of the process hns lmcn widely ucccptcd and modified only in rcsrlcct of u lcw dctrrils. Plocculntion is RSsumcd to occur by 4’brirl&g*y of polymer tnolocuias from onr! particle to iknother. This rcquircs (1) a preliminary Rdsorpt ion ol some sqgncals onto OIICpikrticlc, (2) the pruscnce of oxtcndcd polymer regions strcL&ing out. inlo thu soIi~tion ~“IDoI~s” wilh both ends nttichtwl to the sslmc particle,

more usually,

“tails” wit-h only one end nttnchnl)

or

and (3) some bare patehm

OH the second pzuticle to which the polymer is to nttnch itself. Though It is k3sible t.lmt linkwcs could be cstabIishnl through direct polymer-polymer contacts this is, 011the whole, a rather unlikely ~ircumsfnncc?, since stronl! pnlymcr-polymer intcmctfons rvould militate against thr? kind of open coil configuration which is most ncccssix~y for cffcctivc flocculation. To promote a hi& dt?grcr: of floct ulation it is nccmry lo carefully control the method of polymer addition and the f>rdiminary shearing rcg:mc

so that

floes of suitable size and density arc formed. Unfortunately, in our prcscnt rhrolagical studies there were severe limitations on Chc shear regirnc which could bc imposed. As previously noted, the polymer was added whilst the suspension was being stirred gently (at a shmr rate of order IO s-l) or

more strongly (but still at a slrcar rate of less than 50 s-‘1. In order to compnrc the bchnviour with that studied previously it was considerti desirable to t,rcnt the resulting systems to EI shearing process similar to that used in the sakcoagtilat4 systems, before measurement of the rheological parameters. This shear regime involved a cylindcm-in-cylinder (Couctte) flow at high shear rate in order to axtablish a reproducible size and structure for the flow units.

The polymer is uncharged at this ptI and the added polymer is adsorbed u~~dcr these

the latex is negative. Since ail of conditions, the more or less

sk?ady decrease in ItI is presumably due principally to an outward movement of the plane of shear, although some change in tho ion distribution at the interface cannot be ruled out [X4], A shift of about 12.6 nm is required at the highest polymer levels to account for the obsmvcd drop in I{ 1if no other changes occur. In fact, the adsorption of such a Iargo amount of polymer would have a much more profound effect. The total number of polymer molecules adsorbed per particle, at the highest 5%p/l levels is abut 25. Since the rms end-to-end length is about 100 nm and the arca per polymer molecule is approximately 4n(136)‘/26$ 10’ urn2 the polymer is conrplctcly covering the particle to a depth of about 100 nm at this point. This increase In the effcctivc volume of the particle, assuming that the polymer is nondraining, is quite sufficient to account for the increase in qpr, shown in Pig.3 for pH 10. Indeed, the calculated increase in the radius (97 nm) for V&p/l = 2 from Fig.3 is almost exact.ly the rms extension of the polymer. The elastic floe model is cicarly not applicable if moat of the particlcs are behaving as individuals but Fig.7 shows that the $-potential remains an impotint variable in deWmining tl~o magnitude of the flow parameters. It should be noted that CFP is in lhis cast the ratio

whcrc d is the avuragc polymer layer thickness. At small polymer additions, interactions between the particles arc small, and fR becomes significant only for additions above 1%. The vnluc?sof Da can bc understood in terms of Eq. (3) abovc, if in this case it is taken to bc the point at which individual

Pig.7. Relation between Row properties and t-potent&al at pH 10.The quantity, Cpp is calculated from the volume fraction obtained from l3q. (4). rg: r; D, : 0; CFp: n,

10

particles can be separated by the shear field. In that case 18p is equal to the number of partide-pwticle bonds per unit area of suspension and the value oft used to evaluate FM in Eq. (1) is replaced by (f + d) from Eq. (8). The plot of &/(r + d) then decreases linearly with f2 end has an intercept for g=OofDX 10’s_‘m*. The theoretical value of this intercept requires an estimate of P+- which can be approximated by (PI/Z)~‘~where n is the number of particles per unit volume. Then setting nFA/bq,, X 12 ~3,’= 9 X 10’ s-I m-I and taking A - lo’20 J gives da = 20 nm rvhich is at least of the right order

of magnitude. Despite t-hc geneml comelation displayed in Fig.7 betrveen gqotential flow behaviour it seems that the sedimentation behaviour is not strongly pendent on r: this is not surprising consideringthat the adsorption elf 8 neutral polymer will introduce a steric stabikation component into the action between particles, oven if it lowers the value of ttl. It should also noted that some Roccutation of the system must also be occurring since otherwise the particles would not be expected to settle at all.

and deinterbe

f3chauiolrr at pH 4 At low pH the polymer is strongly positively charged and flocculation can presumably occur by both charge neutralization tlS1 and polymer bridging (141, Small additions of polymer (up to about 1% polymer:p\artick ratio) cause 8 rapid reversal af the sign of the &-potential (Fig.2) and a concomitant change in the q PL rmd T* values similar to that shown by latex systems to rvhich cationic surfactants have been added ( X6). The maximum in the value of the rheological parameters is very close to the point 8t which 5 iS zero and 8 linear r&gE%siOn of r B against 5’ (FIg.8) although showing considerable scatter, gives the same value for the ratio of slope to intercept that has luecn encounkred in the saIt-co;rgulatecl systems l&15, Iti]. This

Fig.8. Relation between rg and cS for systems subjected stirring (0).

l0

Hteak stirring

(a) and strang

11

ratio has previously been interpreted as a measurement of the parameter da, which is the distance at which the force between the particles is a maximum (Eq. (1)). There Is, howvever, an alternative Interpretation which seems more appropriate in this case. The constancy of the ratto merely reflects the fact that the plots of 7 u , Do and C,, aga3nst{* for latex systems always pass through the r’ axis at about 2000 (mV)* and this representsthe point at which the system goes from coagulated to disperse. It is perhaps not surprisingthat even polymer flocculated systems undergo this transition around &I * 45 mV. Likewise, the plot of Do against $‘*for the gently stirred sysCems shows a similar behaviour. For the more strongly st.irredsystem the data is too scattered to draw any conclusion. The floes formed in these systems cannot be treated as non-interacting spheres (Eq. (4)), so no attempt has been made to interpret the plastic viscosity in terms of a CFP value at this pH. It seems that, up to about I% polymer to latex ratio, the behaviour of Ihc polymer is in some respects like that of a cationic surfactant. The interactions in this region behave as though they are purely electrostatic in nature, and there is no direct evidence in thesa measurements of any polymer bridging. Nevetiheless, the long filamantaus appearance of t.ht?flocculum, before it 3ssubjected to high shear rates, suggests that the h3ghly extended polymer chains are dominating the structure and that at this pH it is perhaps more appropriate to regard the latex particles as attached to the polymer along its length rather than vice versa. Such extended chains of particles wit3 be limited in length by the ability of the polymer backbone to withstand the tensile str- exe&d by the shear f3eld. At hig?rerpolymer concentrations, in the gently st3rredsystems there 3s more direct evidence of bridging, even after strong shearing, in the sharp rise in Do dispialed in Fig.& According t.o the elast3cfIoc model, rB and TJP& arc detwmined essentially by the partidepartick interactions, whereas Da 3sdcterm3ned principally by floe-floe interactiuns. ft would seem, from Fig.6 then, that in gently stirred systims with plenty of polymer present, the fiocs that are formed are capable of forming new bridgeato other floes as the shear rate is lowered to around 300 s-‘. This is a good deal lower than the values typ3caUyobserved in saltcoagulated systems md suggests that tha bridges formed in the polymer Socculated SYSterns are somewhat weaker than those involved in the salt-coagulated systems. In the strongly stirred systems most of the plymer may be bedded down too well onk, the particle surface so that icss 3savailahlc for the formation of sat3sfactorybridges from one floe to another or to promote entan&$r?ments. CONCLUSIONS

The ‘*elastic fioc” model of the rheological behaviour of coagulated colloidal sols places considerable emphas3son the significance of the {-potential as a measure of the net attractive interaction between colloidtil particles and floes. In attempting to apply this model to negatively charged latex systems undergoing flocculation with a (potentially) catfonic polymer it is clear that at high pIi, when the polymer is uncharged, the model is of limited value.

12

T h e s m o o t h c h a n g e s in f l o w p r o p e r t i e s (Te, ~p,. a n d Do) w h i c h a c c o m p a n y increa se d p o l y m e r a d d i t i o n s can b e c o r r e l a t e d w i t h c h a n g e s in ~-potential b u t t h e particles a r e p r o b a b l y b e s t t r e a t e d as individuals r a t h e r t h a n floes, w i t h t h e i r size a u g m e n t e d b y t h e a d s o r p t i o n o f p o l y m e r . O n t h e o t h e r h a n d , a t l o w pH, w h e n t h e p o l y m e r is s t r o n g l y positively c h a r g e d , t h e c o r r e l a t i o n b e t w e e n f - p o t e n t i a l a n d r h e o l o g i c a i p r o p e r t i e s is very s t r o n g a n d t h e d a t a a t l o w p o l y m e r c o n c e n t r a t i o n s is v e r y similar t o t h a t o b t a i n e d o n l a t e x s y s t e m s t r e a t e d w i t h c a t i o n i c surfactants. H e r e t h e elastic f l o c m o d e l seems t o w o r k surprisingly well suggesting t h a t a t l o w p o l y m e r a d d i t i o n levels (,~ ] % p o l y m e r : p a r t i c l e r a t i o b y mass), t h e interact i o n s a re d i r e c t l y e l e c t r o s t a t i c in n a t u r e . This d o e s n o t r u l e o u t t h e possibilit y o f bridging b y t h e p o l y m e r , a n d t h e " s t r i n g y " a p p e a r a n c e o f t h e flocs, p r i o r t o s t r o n g shearing, at this p H suggests t h a t a t least s o m e o f t h e p o l y m e r is involved in bridging b e t w e e n particles. T h e precise n a t u r e o f t h e i n t e r a c t i o n s o c c u r r i n g in a p a r t i c u l a r s y s t e m will n o d o u b t b e grea t l y i n f l u e n c e d b y t h e m i x i n g m e t h o d a d o p t e d an d t h e shearing proc e ss t o w h i c h t h e s y s t e m is s u b s e q u e n t l y expt)sed. We o b t a i n e d s o m e e v i d e n c e t o suggest t h a t w h e n a large a m o u n t (:> 1% p/l) o f t h e posi. tively c h a r g e d p o l y m e r is a d d e d , w i t h g e n t l e stirring, t o t h e latex, a n d t h e s y s t e m is s u b s e q u e n t l y sheared at a hi,~h r a t e (~> 1 3 0 0 s-S), t h e flocs d eveloped c a n still f o r m links t o o n e a n o t h e r ( p r o b a b l y b y p o l y m e r bridges) w h e n t h e s h e a r r a t e is l o w e r e d t o a b o u t 3 0 0 s -s or less. Ob v io u s ly m u c h m o r e d e t a i l e d studies o f t h e s e a n d r e l a t e d s y s t e m s will b e n e e d e d in o r d e r t o fully c h a r a c t e r i s e t h e n a t u r e o f t h e b o n d i n g processes involved an d t h e w a y in w h i c h t h e y are i n f l u e n c e d b y t h e shear h i s t o r y o f t h e s y s t e m , ACKNOWLEDGEMENTS We wish t o t h a n k Mr. R i c h a r d M c D o n o g h fo r t h e p o l y m e r a d s o r p t i o n m e a s u r e m e n t s a n d t h e Australian R e s e a r c h G r a n t s C o m m i t t e e fo r t h e i r c o n t i n u e d s u F p o r t . We also t h m , k t h e J o u r n a l referees fo r providing s o m e f r u i t f u l su;,gestions o n i n t e r p r e t a t i o n o f t h e results. R E F E R E N " ~ ~-S

1 B.A. Firth and R.J. Hunter, J. Colloid Interface Sci., 57 (1976) 266. 2

T.G.~5. van d e V e n a n d 1t.3. Hunter, Rheologic& Aeta, 1 6 ( 1 9 7 7 ) 5 3 4 .

3 E.J.W. Verwey and J.q~.O. Overbeek, Theory of Stabitity of Lyophobic CoHolds, I~3sevier. Arns/erdam, 1948, pp. 1 5 2 , ] 60.

4

R.J. Hunter, The Zeta Potential in Colloid Science, Academic Press, London, 1981, p. 3 5 7 .

5 6

7 8

1. KTieger, Adv. Colloid Interface $c1., 3 (1972) 111. R.J. Hunter, Adv. Colloid Interface .~'~., 17 ( 1 9 8 2 ) 197. A. Koter~, K. Furusawa and Y. "]rakedn, KolloSd-Z., 239 (1970) 677. PAl. Wtersema, A.I,. Loeb and Jo'/'h.G. Overbeek, J. Colloid Interlace ScJ., 22 (1966) 78.

13 9 10 ii

13 14 15 16

V.K. Le Mer and ‘F-W. Heaty, Rev. Pure App!. Chem., 13 (1963) 112. R.H. Smellie, Jnr., and V-K. Le Me?, J. Colloid SC~.. 13 (1958) 589. T.W. Healy, and V.K. Ln Mer, J. Phys. Cbem., 66 (1982) 1835. D.E. Brooks, J. Colloid Interface Sci., 43 (1973) 687,700,714. J.W.S. Cbossens, and P, Luner, J. Tech. A-. Pulp Paper Ind., 69 (1976) R.K. Iler, J. Colioid Interface Sci., 37 (197s) 364. J.P. Friend, and R.3. Hunter, 3. Colloid Interface Sci.. 37 (1971) 548. R.J. Hunter, and J. Prayne, 3. Colloid Interface Sci., 71 (1979) 30.

89.