Copolymers, Polyblends, and Composites

10 Process for the Agglomeration of Latex Particles Based on Electrolyte Sensitization

Downloaded by UNIV OF PITTSBURGH on January 20, 2015 | http://pubs.acs.org Publication Date: June 1, 1975 | doi: 10.1021/ba-1975-0142.ch010

HERBERT SCHLUETER Chemische W e r k e Huels AG, Production Department 4, 4370 M a r l , Kreis Recklinghausen, West Germany

Agglomeration rate studies were made with SBR base latex and oxidized poly(ethylene oxide)s (PEO). Included in the investigation were a number of factors which demonstrated that the colloidal basis of this agglomeration is a sensitization to electrolytes. Small amounts of PEO accelerated the rate 10 -fold compared with unsensitized electrolyte agglomeration. This rate difference is attributable to a cooperative action of PEO and anionic emulsifier; there is no necessity to postulate a bridging mechanism. The amount of electrolyte and emulsifier normally needed for the polymerization of SBR latexes may be used for a fast agglomeration, and some technologically important factors may be changed at will without any resultant coagulum formation. This makes the process versatile for large scale operations. 7

R

eactions based on interactions between hydrophilic polymers and hydrophobic colloids are common in nature and i n industry. In biological systems, many reactions and processes have been interpreted i n terms of polymer-colloid interactions (1, 2, 3). I n various branches of industry, hydrophilic polymers are used as stabilizers or destabilizers (4, 5, 6). Especially in latex technology, hydrophilic polymers are used as agglomeration activators (7,8). I n this case, agglomeration means aggregation and coalescence of small particles into larger ones, as required for fluidity at high latex concentrations. Often stabilization and destabilization may occur i n the same system. A t high polymer concentrations the system may be strongly stabilized whereas at low concentrations the dispersion may flocculate. I n the latter case, a distinc tion can be made between (a) adsorption flocculation (i.e>., flocculation b y polymer only) and (b) sensitization (i.e., in addition to polymer some electro lyte is needed to ensure complete flocculation). The mechanism of sensitization is far from clear. Basically, there are different concepts regarding the roles of polymer and electrolyte i n this process. In one case, for example, the effective charge density on particle surface is lowered b y polymer adsorption so that the colloidal particle is made sensitive to electrolyte, and it can be flocculated even at lower electrolyte concentrations than i n the absence of polymer (9). I n the other case, b r i d g i n g between the 99

In Copolymers, Polyblends, and Composites; Platzer, N.; Advances in Chemistry; American Chemical Society: Washington, DC, 1975.


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particles through segments of the polymer molecule is the deciding factor, a n d a certain critical amount of electrolyte is needed (10). T h e electrolyte reduces the thickness of the diffuse double layer, and this could make the polymer more effective since additional lengths of polymer molecule w o u l d then be available for bridging. A l o n g these lines, the agglomeration of latex particles promoted b y poly (methyl v i n y l ether) ( P V M ) has been discussed (11). In this connection, the question arises h o w synthetic rubber particle agglomeration carried out w i t h oxidized poly(ethylene oxide) ( P E O ) (12) can bë interpreted i n colloidal terms. This agglomeration m a y also be defined in terms of p o l y m e r - c o l l o i d reactions as a flocculation occurring at l o w polymer concentrations w h i c h , however, is restricted to the colloidal particle size range (13). T h e reason for termination of agglomeration, short of latex particle coagulation, is not only of interest because of the common occurrence of poly mer-colloid interactions i n nature and industry, as has already been mentioned, but because it is of major importance especially i n latex technology for further developments i n the field of controlled agglomeration w i t h hydrophilic polymers. Attempts to answer these questions by quantitative rate studies are reported here. Experimental Materials.

STYRENE-BUTADIENE

RUBBER

(SBR)

LATEX.

SBR

latex was

prepared b y redox emulsion polymerization using ( i n p a r t s ) : butadiene ( 6 9 ) and styrene ( 3 1 ) at 6 ° - 4 0 ° C (pinane hydroperoxide/sodium formaldehyde sulfoxylate/Fe as initiator) i n the presence of potassium oleate ( 2 . 7 ) inorganic electrolytes ( 0 . 4 5 ) as polymerization aids, and demineralized water ( 1 3 5 ) until a conversion of 7 0 % was achieved. Residual monomers were then removed. T h e latex had the following physical properties: specific surface, 1 0 6 m / g ; soap coverage, 2 6 % ; and particle concentration, 5 . 3 X 1 0 / m l [corresponding to 3 2 . 5 % latex total solids ( T S ) ] . T h e size distributions of the latex particles are plotted i n Figure 1 . T h e size scale i n a l l figures showing particle size distri butions is logarithmic for convenience i n presentation since the agglomerated latexes have relatively large size ranges. In addition, S B R latexes w i t h less emulsifier and inorganic electrolyte as well as some w i t h larger particles and modified particle size distributions were also prepared for controlled variations of these factors. ++

2

1 5

CARBOXYLIC STYRENE-BUTADIENE

(SB)

LATEX.

T h e carboxylic latex was

prepared b y emulsion polymerization at 6 0 ° C using ( i n parts) : butadiene ( 4 0 ) , styrene ( 5 7 . 5 ) , and acrylic acid ( 2 . 5 ) i n the presence of demineralized water ( 1 3 8 ) , C - s u l f o n a t e ( 0 . 5 ) as emulsifier, and tertiary dodecyl mercaptan ( 0 . 5 ) and ammonium persulfate ( 0 . 5 ) as initiator. P O L Y ( E T H Y L E N E O X I D E ) . P E O was prepared using polyethylene glycols of commercial grade (12). P O T A S S I U M O L E A T E . Pamolyn 1 0 0 , a product of Hercules Powder L t d . , was used. 14

INORGANIC E L E C T R O L Y T E .

T h e concentration of potassium ions, [ K ] , i n +

the aqueous latex phase was varied using potassium chloride and potassium sulfate (analytical reagent grade) i n a 2 : 1 ratio. Methods.

DETERMINATION

OF AGGLOMERATION RATE.

The

kinetics of

P E O agglomeration may be described i n a w i d e initial range (up to of total agglomeration) b y all three forms of Smoluchowski's equation, the reciprocal particle number (1/N), the mean particle volume (ν), reciprocal cube of the particle surface ( 1 / Σ ) vary linearly w i t h time 3


50-70% i n which and the (t) (14,

10.

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101

Latex Particle Agglomeration

50J

40 J


30J

VOLUME DISTRIBUTION NUMBER DISTRIBUTION

20J

I0J

1

1—r-i-r

5 10 SO DIAMETER D (Aid )

τ 100

2

Figure 1. Particle size distributions of base latex N j , number of particles with diameter D i ; N i · D i , volume of particles with diameter D i ; I N j , sum of total N i ; and Σ N i · Dr*, sum of total N j D i * J

15, 16). l/N , v , and 1 / Σ are the corresponding values of the base latex when t — 0. Φ represents the volume fraction of the latex particles. T h e 0

0

0

3

l/N

-

l/No

ν — v

0

18πΦ

3

=

=

k

N

(1)

· t

(2)

kv - t

18χΦ

3

(3)

V3

equations can be used to determine the agglomeration rate constants k , k~, and /c . Determination of 1 / N or υ b y electron microscopy, ultracentrifugation (17), or an alginate creaming method (18, 19) is very time-consuming. Therefore, soap titration to m i n i m u m surface tension (20) was used almost exclusively for the determination of Σ, and Equation 3 was used i n these experiments. Figure 2 shows that 187τΦ Σ increased linearly w i t h time over a relatively wide initial range. The stability factor is defined as: N

s

3

-3

Α-Σ ο


(4)


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'6

TIME (MINS) Figure 2.

Variation of reciprocal cube of particle surface (Σ~ ) with time at 40°C 3

Particle surface (Σ) in cm*/ml latex, and volume of dispersed phase (Φ) in cm*/ml latex

where k^ is the fast Smoluchowski rate constant assuming that each particle collision leads to agglomeration because of diffusion. This can be derived from: 0

k

r

ο

(5)

—

where k = Boltzmann constant, Τ = absolute temperature, a n d η = viscosity of water. Unless stated otherwise, the agglomeration experiments were carried out in a temperature chamber at 4 0 ° C a n d 3 2 . 5 % T S using 0 . 1 % P E O / T S . Samples were collected at fixed intervals, and the reaction was terminated b y adding the samples to sufficient potassium oleate solution to decrease the sur face tension of the latex to about 4 0 dyne/cm or b y diluting w i t h water to about 6 % T S . T h e particle surface ( Σ ) was then determined b y soap titration so that rate constant ( & ) and stability factor could be calculated from E q u a tions 3 , 4 , and 5 . H i g h agglomeration rates i n the region of log W < 4 . 3 were determined by a rapid flow method, referred to as quenched flow mode (21), using an apparatus specially constructed for this purpose (22). S U R F A C E T E N S I O N . Surface tension as a function of potassium oleate con centration i n the presence and absence of P E O was measured b y the R i n g detachment method at 2 5 ° C . T h e values were corrected b y the method of Harkins and Jordan (23). Σ

rrpt


10.

SCHLUETER

103


DETERMINATION

OF PARTICLE

SIZE DISTRIBUTION.

Particle size distribu

tions of base latex and agglomerated latex were determined b y evaluating electron micrographs {24).


Results Particle Size Distribution Changes. T h e number frequency distribution of particles for the base and end latexes are plotted i n Figure 3. N o particles i n the end latex formed b y agglomeration had a diameter larger than 9600 A . Furthermore, particles of 1300-9600 A constituted only about 1 . 2 % of the total number of particles. The small particle size (200-400 A ) that had consti tuted more than 5 % i n the base latex disappeared completely. T h e peak of the distribution curve shifted from about 380 A to 700 A . T h e volume frequency distribution of latex particles after agglomeration and that of the base latex are plotted in Figure 4. Large particles 1300-9600 A in diameter constituted 6 3 % of the total particle volume (cf. Figure 3 ) . Particle size distributions of samples collected during agglomeration demonstrate that mean particle size increased steadily, though it is interesting that many of the larger particles were already formed during early stages of

50J

DIAMETER D (ÀKT) Figure 3.

Number distributions of particles before and after agglomeration , base latex; and

, end latex


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AND

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50J

Figure 4.

Volume distributions of particles , base latex; and

, end latex

agglomeration. Thus, the larger particles may serve as targets for the smaller ones that act as bullets. Factors Affecting P E O Agglomeration. B y using Equation 3, it is easily possible within a relatively short time to study the effects of the many factors w h i c h usually play a part i n polymer-colloid reactions and w h i c h have to be considered i n P E O agglomeration, too. These factors are tabulated i n Table I according to reactants and reaction conditions. Table I. SBR

Factors in P E O Agglomeration

Base Latex

Inorganic electrolyte (amount and type) Anionic emulsifier (amount and type) Particles (concentration, size, and size distribution) Polymer (butadiene/styrene ratio, molecular wt, residual monomers, etc.)

PEO

Concentration Polymer (molecular wt, structure [e.g. degree of branching], hydrophobic groups, etc.)

Reaction Conditions

Temperature Agitation Operation (batchwise or continuous) Method of mixing P E O and latex


10.

SCHLUETER

105


Inorganic Electrolyte and Anionic Emulsifier. T h e plot of log W vs. log [ K ] is a straight line (Figure 5 ) . T h e slope of the line indicates that the rate of agglomeration varied w i t h the 5th to 6th power of [ K ] . T h e de pendence, however, could be followed experimentally only as far as log W ~ 4.3 because at this point agglomeration time was already reduced to a few seconds. A special flow apparatus was used i n order to follow agglomeration over a wider [ K ] range, including the range where agglomeration rate becomes independent of [ K ] (see Figure 6 ) . This occurs w h e n each particle collision leads to an agglomeration, and the time of agglomeration, which at this point was already less than 10~ sec, cannot be reduced further. Nevertheless, the rate constant (k^ ) can still be determined experimentally. It was found that &Σο = 0.8 · 1 0 " cm /sec whereas the calculated value is 1.4 · 10" /sec. Consequently, as a rule the values of log W w o u l d be 0.24 lower if we had used the experimentally determined value of £ . If the straight line shown in Figure 6 is extrapolated to [ K ] = 0, agglomeration time increases to about 200,000 years. It is very impressive that this huge time can be reduced to about 10" sec b y increasing [ K ] in the aqueous phase from 0 to 2.4%. These findings agree w i t h the relations w h i c h were determined i n electro lyte coagulation studies, and they can be substantiated by the D e r j u i n - L a n d a u Vervey-Overbeek ( D L V O ) theory (25, 26, 27). Particle agglomeration of a commercial S B R latex using electrolyte only is also possible under certain conditions without coagulum formation (28). T h e emulsifier content was i n creased from 4.41 to 6.4% T S i n order to avoid coagulum formation and to cover a wider [ K ] range. T h e findings for agglomeration obtained with and expt

+

+

€œpt

+

+

3

0


12

3

12

expt

Σ ο

+

3

+

+

ΊΟ

Ί 2

ΊΛ

'IB

"2.0 "2.2

"18

L0G[K*](M EQUIV/L)

Figure 5. Log W\. t and log agglomera tion time vs. log [K ] xp

+

[K+] —-- potassium ion concentration of neutral electrolyte in mequi\ /I


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COPOLYMERS,

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i

Ί.6

Ί.8 "2J0 "2.2 "24 "2J5 "2J8 LOGIK*] (M EQUIV7L)

Figure 6. Log W x and Zog agglomeration time vs. Zog in £/ie range of lower stability factors Wexpt and shorter agglomeration times e

Ί2

U

Ί6

Ί8

Pt

' U '16 '18 '2J0 '22 "2Λ L0G[K^](MEQUIV7L)

·20 '22 '2Λ

LOGIW (M EQUIV./L)

Figure 7. ,ννζϊΛ

Log W PEO; and

e i p

t vs. Zog , without PEO

Figure 8.

Log Wexpt vs. Zog [ K ]

, anrf , without PEO. % Κ oleate/TS latex: A , 2.4; • , 4.46; and ·, 6.4.


+

10.

SCHLUETER

107

Latex Τ article Agglomeration

without P E O are presented i n Figure 7. The distance between the two straight lines indicates that P E O accelerated the agglomeration rate 10 -fold, and that at constant rates [ K ] can be reduced to almost 1/20. W h e n the emulsifier content of latex was changed systematically, a funda mental difference between agglomerations with and without P E O was apparent (see Figure 8 ) . T h e distance between the straight lines, i.e. the acceleration by P E O , decreased considerably as emulsifier content was reduced. 7

+

Log W is plotted against l o g Κ oleate concentration i n Figure 9. Dependence was not linear. In the range of greatest dependence, the agglom eration rate varied even with the 7th power of [ K oleate]. Therefore, i n this range the dependence on oleate concentration is even more pronounced than that on inorganic electrolyte concentration. T h e plot also suggests that the curvature tends to become sigmoidal with increasing range of potassium oleate concentration.


expt

Regarding the type of inorganic electrolyte, there was no marked differ ence i n effectiveness between the monovalent cations, but preliminary experi ments using S B R latexes with emulsifiers which, i n contrast to oleate, do not form insoluble salts with divalent cations, demonstrated that i n this case P E O was less effective. Findings i n earlier sensitization experiments were similar (29, 30). A s to the type of anionic emulsifier, a slight correlation between emulsifier surface activity and agglomeration rate was observed. Particle Concentration. T h e effect of particle concentration was essen tially that which can be expected from a second order reaction (time for half-agglomeration was inversely proportional to the number of particles origi nally present). However, at T S > 3 5 % and higher viscosities and T S < 1 5 % of base latex, the reaction was faster or slower, respectively, than expected. Particle Size. T h e effect of particle size on agglomeration rate at constant electrolyte concentration corresponded roughly to that which w o u l d be expected from Smoluchowski's rate equations, i.e., no effect was observed. Figure 10 shows that i n the plot of log W . vs. log [ K ] , the slope of the straight line was not dependent on particle size. Such dependence could not be confirmed although it is predicted b y theoretical calculations ( 2 5 ) . Interestingly, this finding agrees with that i n electrolyte flocculation experiments using poly styrene latex dispersions of various particle sizes (31), i.e., an increase in slope with increase i n particle size was not observed experimentally. It should be mentioned that attempts were made recently to eliminate this discrepancy by more refined calculations (32). Nevertheless, the effect of base latex particle size on the TS/viscosity relation of the end latex was marked. As base latex particle size increased, T S decreased when the latex was concentrated after agglomeration to a viscosity of 1200 c P . CJ pf

+

Particle Size Distribution. T h e effect of particle size distribution was roughly in accordance w i t h the theory of Mueller who extended Smoluchowski's theory to polydisperse systems (33). According to this theory, a particle diameter ratio of 1:10 i n the polydisperse system reduces the time of halfcoagulation b y the ratio 1.0 to 0.4. In contrast to other polymers proposed for agglomeration, it is unimpor tant whether or not the base latex has a fairly uniform particle size in order to avoid coagulum formation (34). W i t h base latexes containing large amounts (e.g., 40-50 w t % ) of large particles ( > 1200 A ) , no coagulum at all was obtained after agglomeration.


108

COPOLYMERS,

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ΎΙ

ΊΟ

II10-


9_

5-

3. Γ

Ϊ4

Γ

Ϊ6

Ίβ

S

Ί 2 'ΙΛ '16 '18 '20 '22

LOG [Κ OLEATEKM MOL/U

Figure 9. Log W

M p

[K oleate] in mmol/l;

, vs. log [K oleated] [K+] = 34.8 mequiv/l

LOGIWIMEQUIWL)

Figure 10.

Log W pt vs. Zog eX

Particle diameter in angstroms: X , 560; O, 760; and ·, 820

S B R Latex Polymer. A l l factors w h i c h make particle coalescence easier should also affect the agglomeration rate. Therefore, the second order transi tion temperature of S B R latex polymer should be low. Thus it is understandable that various factors (e.g., the ratio of butadiene to styrene, residual monomer, and molecular weight of polymer) are of importance i n agglomeration. P E O Concentration. A plot of l o g W against l o g P E O concentration is presented i n F i g u r e 11. T h e slope of the straight line indicates that at low [ P E O ] the rate varied w i t h the 1.5th power of P E O concentration. After reaching a m i n i m u m , the curve rose sharply when P E O concentration was increased further. P E O P o l y m e r . There can be little doubt that polymer properties—e.g. molecular weight, structure (degree of branching and slight crosslinking), and content of hydrophobic groups—are extremely critical factors, and they have to be adjusted optimally. However, m u c h additional work is needed i n order to describe these factors quantitatively. This also applies to the role of helix conformations w h i c h were recently detected b y calorimetric measurements using aqueous polyethylene glycol solutions ( 3 5 ) . Temperature. A s regards dependence on temperature, there was an interesting parallel to the anomalous temperature effects (36) observed i n many enzyme reactions (above 35°C, the rate increased b v 4 0 % / 1 0 ° C and below 35°C b y 8 0 - 1 0 0 % / 1 0 ° C ) . Agitation. In contrast to other polymers proposed for agglomeration, i n this system agitation conditions d i d not have any effect on agglomeration rate and coagulum formation. This agrees w i t h Smoluchowski's calculations, namely that agitation can affect coagulation rate only when particle radius is larger than 1 0 A . A s the latex particle radius before and after agglomeration was expt

4


10.

SCHLUETER


109

far smaller than this value, agitation conditions h a d practically no effect on agglomeration. Batchwise or Continuous Operation. It is also unimportant whether the agglomeration is carried out batchwise or continuously. This can be demon strated by conducting the agglomeration i n an agitated vessel or in a special flow apparatus like that mentioned above. There was good agreement of agglomeration rate i n both cases. It was, of course, possible to make the com parison only i n the higher W . range where agglomeration i n an agitated vessel can still be followed and measured. M e t h o d of M i x i n g P E O and Latex. Again in contrast to other polymers (20, 34 ), no effect of mixing method on either agglomeration rate or coagulum formation was detectable in this system. Whether or not it has noticeable effect on particle size distribution remains to be elucidated b y evaluating electron micrographs.


VJ pt

Discussion Mechanism. There can be no doubt from the above findings that an intense sensitization does exist in P E O agglomeration. This is also illustrated by the plot of arbitrarily defined agglomeration concentration of monovalent cations vs. P E O concentration (see Figure 1 2 ) . T h e curvature is characteristic of sensitization. A minimum of agglomeration concentration and a change from sensitization to protection at higher P E O concentration are apparent. The log W /\og [ K ] relations reveal quite clearly that, now as before, the reaction is governed by inorganic electrolyte or, more exactly, by counter ions i n an unadulterated manner though at considerably lower concentrations. Bridging as a decisive factor, as was assumed i n P V M agglomeration ( 1 2 ) . can therefore be excluded. Obviously the role of P E O is to destabilize the particle b y adsorption on the particle surface and to make the particle sensitive erpt

+


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>

K? ο


Ό

3

Copolymers, Polyblends, and Composites

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