Stabilization of Aqueous Pesticidal Suspensions by Graft Copolymers

2 Stabilization of Aqueous Pesticidal Suspensions by Graft Copolymers and Their Subsequent Weak Flocculation by Addition of Free Polymer 1

D. HEATH and TH. F. TADROS

Downloaded by UNIV LAVAL on July 11, 2016 | http://pubs.acs.org Publication Date: June 5, 1984 | doi: 10.1021/bk-1984-0254.ch002

ICI Plant Protection Division, Jealott's Hill Research Station, Bracknell, Berkshire, RG12 6EY, United Kingdom R. D. KNOTT and D. A. KNOWLES ICI Plant Protection Division, Works Experimental Department, Yalding, nr Maidstone, Kent, United Kingdom Graft copolymers as dispersing agents for pesticidal suspensions concentrates have been investigated using ethirimol (a fungicide) and a copolymer of polymethyl methacrylate-methacrylic acid/methoxypolyethylene -oxide methacrylate. The results showed that highly concentrated dispersions (with a volume fraction > 0.6) can be obtained. This agent is strongly adsorbed on the particle surface and provides a steric barrier preventing flocculation. The s t e r i c a l l y stabilised dispersions produced can be weakly flocculated by the addition of free (non-adsorbing) polymer such as poly(ethylene oxide). Weak flocculation occurs above a c r i t i c a l free polymer concentration which decreases with increase of molecular weight, M , of the free polymer. The weak flocculation produced in concentrated (55% w/w) dispersions was investigated using rheological measurements. Steady state shear stress-shear rate curves were established and the yield value, τ , was obtained by extrapolation of the linear portion of the curve. Moreover, the yield value was also directly obtained by applying a series of successive stresses of equal increments to the suspension and recording the response u n t i l flow occurred. The results showed that above the flocculation point τ increases with increase of free polymer volume fraction, Ф . This increase took place at a critical, Ф value which decreased with increase of M . The rheological results were interpreted in terms of the energy-minimum, G , produced in the free energy-distance W

β

β

p

p

W

min

1

Author to whom correspondence should be addressed. 0097-6156/84/0254-0011 $06.00/0 © 1984 American Chemical Society Scher; Advances in Pesticide Formulation Technology ACS Symposium Series; American Chemical Society: Washington, DC, 1984.

12

ADVANCES IN PESTICIDE FORMULATION TECHNOLOGY

curve i n the presence of free polymer. Good correlation was obtained between the increase i n the depth of G and the increase i n τ . Sediment height and redispersion experiments were also consistent with the weak flocculation phenomenon, showing also good correlation with the rheological data.


min

β

The f o r m u l a t i o n o f a q u e o u s c o n c e n t r a t e d s u s p e n s i o n s o f p e s t i c i d e s r e q u i r e s t h e u s e o f p o w e r f u l d i s p e r s i n g a g e n t s , w h i c h keep t h e particles i n a deflocculated state. This i s the case p a r t i c u l a r l y when h i g h v o l u m e f r a c t i o n s (> 0.4) o f t h e s o l i d a r e required. F l o c c u l a t i o n produces a r a p i d increase i n v i s c o s i t y thus causing problems during p r e p a r a t i o n . Moreover, s t r o n g f l o c c u l a t i o n i s u n d e s i r a b l e s i n c e i t may c a u s e s e v e r e t h i c k e n i n g on s t o r a g e a n d r e d u c e s t h e s p o n t a n e i t y o f d i s p e r s i o n on d i l u t i o n o f t h e s u s p e n s i o n . S e v e r a l a g e n t s may be u s e d t o o b t a i n a d e f l o c c u l a t e d suspension, i . e . , a s t a b l e suspension i n t h e c o l l o i d sense ( 1 ) . T h e s e may be c l a s s i f i e d i n t o i o n i c a n d n o n i o n i c s u r f a c e a c t i v e agents, n o n i o n i c macromolecules and p o l y e l e c t r o l y t e s . Ionic s u r f a c t a n t s produce d e f l o c c u l a t e d suspensions as a r e s u l t o f t h e d o u b l e l a y e r r e p u l s i o n p r o d u c e d b y t h e a d s o r b e d c h a i n s . However, these a r e seldom s u i t a b l e f o r p r o d u c i n g c o n c e n t r a t e d suspensions s i n c e double l a y e r o v e r l a p l e a d s t o an i n c r e a s e i n t h e v i s c o s i t y a t m o d e r a t e l y h i g h volume f r a c t i o n s . M o r e o v e r , t h e s e systems a r e s e n s i t i v e t o e l e c t r o l y t e s a n d t h e s u r f a c t a n t may n o t b e s t r o n g l y a d s o r b e d on t h e p a r t i c l e s u r f a c e s . N o n - i o n i c s u r f a c t a n t s c a n be u s e d as d i s p e r s i n g agents as a r e s u l t o f s t e r i c r e p u l s i o n between the adsorbed l a y e r s . Provided the chains are w e l l solvated, s t r o n g s t e r i c r e p u l s i o n e n a b l e s one t o o b t a i n d e f l o c c u l a t e d systems. However, t h e s e s y s t e m s may s u f f e r f r o m d e s o r p t i o n o f t h e c h a i n s w h i c h a r e i n many c a s e s o n l y w e a k l y a d s o r b e d on t h e p a r t i c l e s u r f a c e s . In theory t h e best agents f o r p r o d u c i n g d e f l o c c u l a t e d suspensions a r e those polymers o f t h e b l o c k and g r a f t type (2). T h e s e a g e n t s a r e d e s i g n e d t o h a v e two m a i n g r o u p s A a n d B, where A has a s t r o n g a f f i n i t y t o t h e p a r t i c l e s u r f a c e ( i e . s t r o n g l y and i r r e v e r s i b l y adsorbed and b e i n g i n s o l u b l e i n t h e continuous medium) a n d Β i s a g r o u p t h a t p r o v i d e s t h e s t e r i c s t a b i l i s a t i o n , b e i n g h i g h l y s o l v a t e d b y t h e c o n t i n u o u s medium a n d o f s u f f i c i e n t l e n g t h t o p r o v i d e a s t e r i c b a r r i e r . A-B, B-A-B b l o c k a n d A - B g r a f t c o p o l y m e r s may be u s e d f o r t h i s p u r p o s e . The a n c h o r i n g A n

g r o u p c a n be a n i n s o l u b l e h y d r o p h o b i c g r o u p s u c h a s p o l y s t y r e n e or polymethymethacrylate, which has a s t r o n g a f f i n i t y t o t h e h y d r o p h o b i c p e s t i c i d e p a r t i c l e , w h e r e a s Β c a n be a w a t e r s o l u b l e c h a i n such as p o l y ( e t h y l e n e o x i d e ) . T h i s h y p o t h e s i s has been t e s t e d i n t h e p r e s e n t i n v e s t i g a t i o n by u s i n g a g r a f t copolymer o f

Scher; Advances in Pesticide Formulation Technology ACS Symposium Series; American Chemical Society: Washington, DC, 1984.

2.

HEATH ET AL.

13

Stabilization of Aqueous Suspensions

polymethylmethacrylate-methacrylic acid/methoxy p o l y e t h y l e n e oxide methacrylate. The p o l y m e r i s t h e r e f o r e , a "comb" t y p e w i t h p o l y e t h y l e n e o x i d e c h a i n s ( t e e t h ) . As an e x a m p l e o f a p e s t i c i d e , e t h i r i m o l ( a f u n g i c i d e m a n u f a c t u r e d by I C I ) was used. The a n c h o r i n g o f t h e c h a i n t o t h e p a r t i c l e s was a s s e s s e d u s i n g a d s o r p t i o n / d e s o r p t i o n i s o t h e r m s , w h e r e a s i t s d i s p e r s i n g power was e v a l u a t e d f r o m measurement o f v i s c o s i t y a s a f u n c t i o n o f v o l u m e f r a c t i o n of the p a r t i c l e s .


w

w

Although the production of h i g h l y d e f l o c c u l a t e d suspensions i s a primary o b j e c t i v e f o r formulation of suspension c o n c e n t r a t e s , these systems tend t o s e t t l e under g r a v i t y forming d i l a t a n t sediments ( c l a y s ) . The l a t t e r must be p r e v e n t e d e i t h e r by c o n t r o l l e d f l o c c u l a t i o n o r by t h e a d d i t i o n o f a s e c o n d d i s p e r s e p h a s e t o t h e c o n t i n u o u s medium ( 1 ) . One m e t h o d w h i c h may be a p p l i e d t o s t e r i c a l l y s t a b i l i s e d d i s p e r s i o n s , i s t o a d d a f r e e ( i e . n o n - a d s o r b i n g ) p o l y m e r t o t h e c o n t i n u o u s medium. R e c e n t s t u d i e s i n o u r l a b o r a t o r y (3) u s i n g p o l y s t y r e n e l a t e x d i s p e r s i o n s s t a b i l i s e d by t h e above c o m b d i s p e r s i n g agent have shown t h a t p o l y m e r s s u c h a s p o l y ( e t h y l e n e o x i d e ) i n d u c e weak f l o c c u l a t i o n above a c e r t a i n c r i t i c a l c o n c e n t r a t i o n o f f r e e p o l y m e r w h i c h was d e p e n d e n t on t h e m o l e c u l a r w e i g h t o f t h e c h a i n . M

n

T h i s phenomenon h a s b e e n a p p l i e d t o t h e p r e s e n t p e s t i c i d a l s u s p e n s i o n and t h e r e s u l t s o b t a i n e d ( u s i n g r h e o l o g i c a l and sediment volume e x p e r i m e n t s ) a r e g i v e n i n t h e p r e s e n t p a p e r . Experimental Materials Ethirimol, crystalline

solid

a f u n g i c i d e m a n u f a c t u r e d by

(density =

1.21

g cm"

I C I , was

) w h i c h was

used

a

white

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received. The "comb" d i s p e r s i n g a g e n t was a g r a f t c o p o l y m e r o f polymethylmethacrylate-methacrylic acid (methoxypolyethylene o x i d e m e t h a c r y l a t e ) s u p p l i e d by I C I P a i n t s D i v i s i o n ( S l o u g h ) a n d u s e d a s r e c e i v e d . The e x a c t m o l e c u l a r w e i g h t o f t h e p o l y m e r i s n o t known, b u t i t i s e x p e c t e d t o be i n t h e r e g i o n o f 20-30,000 ( a s i n d i c a t e d by I C I P a i n t s D i v i s i o n ) . The M of the p o l y e t h y l e n e c h a i n s was 750. T h r e e p o l y e t h y l e n e o x i d e (PEO) s a m p l e s w i t h n o m i n a l m o l e c u l a r w e i g h t s 20,000, 35,000 a n d 90,000 were u s e d . These were s u p p l i e d by F l u k a , H o e c h s t , a n d U n i o n C a r b i d e r e s p e c t i v e l y , and u s e d as r e c e i v e d . w

A d s o r p t i o n Isotherms I n t h e s e e x p e r i m e n t s a "comb" w i t h a s i m i l a r s t r u c t u r e was u s e d . The e t h i r i m o l u s e d was a r e c r y s t a l l i s e d m a t e r i a l which had been ground ( u s i n g a ^ C o f f e e m i l l ) t o p r o d u c e a p o w d e r w i t h s u r f a c e a r e a 0.29 m g" (as m e a s u r e d b y BET K r a d s o r p t i o n ) . A b o u t 1 gm o f s o l i d was a c c u r a t e l y w e i g h e d i n t o 2oz w i n c h e s t e r s a n d t h e n 25cm^ s o l u t i o n s o f t h e d i s p e r s i n g a g e n t , c o v e r i n g a c o n c e n t r a t i o n o f 0-1000 ppm were a d d e d . The p o w d e r was d i s p e r s e d


14


u s i n g a s o n i p r o b e and t h e n l e f t s t i r r i n g o v e r n i g h t t o r e a c h e q u i l i b r i u m . Ten cm o f e a c h d i s p e r s i o n was t h e n c e n t r i f u g e d f o r h a l f an h o u r a t 4000 g a n d t h e n f o r a n o t h e r h a l f an h o u r a t 25,000 g. The c o n c e n t r a t i o n o f t h e p o l y m e r i n t h e s u p e r n a t a n t l i q u i d was d e t e r m i n e d u s i n g a c o l o r i m e t r i c m e t h o d b a s e d on complexation with I l (4)· The p r o c e d u r e h a s b e e n d e s c r i b e d b e f o r e ( 5 ) . D e s o r p t i o n was i n v e s t i g a t e d by r e d i s p e r s i n g t h e s e d i m e n t e d p a r t i c l e s i n t o d i s t i l l e d water, r e c e n t r i f u g i n g and a n a l y s i n g f o r any d e s o r b e d p o l y m e r . +

K


2

Preparation of Suspensions A 66.8% w/w s u s p e n s i o n was p r e p a r e d by s l o w l y a d d i n g t h e powder t o t h e a q u e o u s s o l u t i o n o f t h e "comb" s t a b i l i s e r ( 2 % w/w) while s t i r r i n g . C a r e was t a k e n t o a v o i d a i r being entrapped i n the suspension. The l a t t e r was t h e n m i l l e d u s i n g a Dyno m i l l ( e x . W i l l y . A. B a c h o f e n , B a s l e , S w i t z e r l a n d ) a n d then l e f t standing while being s t i r r e d . I n some c a s e s , t h e p a r t i c l e s i z e d i s t r i b u t i o n o f t h e r e s u l t i n g s u s p e n s i o n was d e t e r m i n e d u s i n g an o p t i c a l d i s c c e n t r i f u g e . T y p i c a l l y , the s u s p e n s i o n g a v e a d i s t r i b u t i o n t h a t was >50% b e l o w 1μπι i n d i c a t i n g adequate m i l l i n g . F o r r h e o l o g i c a l measurements ( s e e below) samples o f t h e s u s p e n s i o n c o n c e n t r a t e s were m i x e d w i t h PEO s o l u t i o n s t o g i v e 55% w/w e t h i r i m o l , w h i l e v a r y i n g t h e c o n c e n t r a t i o n o f PEO. For v i s c o s i t y - v o l u m e f r a c t i o n c u r v e s a 60% w/v s u s p e n s i o n was p r e p a r e d by m i l l i n g u s i n g 3% o f t h e "comb" s t a b i l i s e r . The r e s u l t i n g s u s p e n s i o n was f u r t h e r c o n c e n t r a t e d by c e n t r i f u g a t i o n and t h e sediment d i l u t e d w i t h t h e s u p e r n a t a n t l i q u i d t o g i v e a range o f volume f r a c t i o n s . Sediment volume and r e d i s p e r s i o n F o r sediment volume e x p e r i m e n t s a 50% w/v s u s p e n s i o n was p r e p a r e d u s i n g a 2% w/w o f t h e p o l y m e r . 5g o f t h e r e s u l t i n g s u s p e n s i o n was a d d e d t o 5 m l s o l u t i o n s o f PEO t o c o v e r a wide c o n c e n t r a t i o n and the r e s u l t i n g s u s p e n s i o n p l a c e d i n s t o p p e r e d c y l i n d e r s and k e p t i n c o n s t a n t t e m p e r a t u r e c a b i n e t s (25 ± 1 ° C ) . The s e d i m e n t h e i g h t was f o l l o w e d w i t h t i m e f o r s e v e r a l weeks u n t i l e q u i l i b r i u m was r e a c h e d . At t h i s p o i n t the t u b e s were m e c h a n i c a l l y i n v e r t e d e n d - o v e r - e n d a n d t h e number o f r e v o l u t i o n s r e q u i r e d f o r r e d i s p e r s i o n was n o t e d . R h e o l o g i c a l measurements Two i n s t r u m e n t s were u s e d t o i n v e s t i g a t e the rheology of the suspensions. The f i r s t was a Haake R o t o v i s k o m o d e l RV2(MSE S c i e n t i f i c I n s t r u m e n t s , Crawley, S u s s e x , E n g l a n d ) f i t t e d w i t h an MK50 m e a s u r i n g h e a d . This i n s t r u m e n t was u s e d t o o b t a i n s t e a d y s t a t e s h e a r s t r e s s - s h e a r r a t e c u r v e s . From t h e s e c u r v e s i n f o r m a t i o n c a n be o b t a i n e d on t h e v i s c o s i t y as a f u n c t i o n o f s h e a r r a t e . The y i e l d v a l u e may be o b t a i n e d by e x t r a p o l a t i o n o f t h e l i n e a r p o r t i o n o f t h e s h e a r s t r e s s - s h e a r r a t e curve t o zero shear r a t e . The p r o c e d u r e h a s been d e s c r i b e d b e f o r e ( 3 ) .


2.

HEATH ET AL.

15


The s e c o n d i n s t r u m e n t was a D e e r r h e o m e t e r (model PDR881, I n t e g r a t e d P e t r o n i c Systems L t d . , London) f i t t e d w i t h c o n c e n t r i c cylinder platens. T h i s i n s t r u m e n t was u s e d t o m e a s u r e o f t h e y i e l d v a l u e by a p p l y i n g a s e r i e s o f s t r e s s v a l u e s o f e q u a l i n c r e m e n t s and r e c o r d i n g t h e r e s p o n s e u n t i l f l o w o c c u r r e d .


Results Adsorption Isotherm F i g . 1 shows t h e a d s o r p t i o n isotherm ( o b t a i n e d a t room t e m p e r a t u r e 2 0 ± 2 ° C ) o f t h e "comb" s t a b i l i s e r on ethirimol. The i s o t h e r m i s o f t h e h i g h a f f i n i t y t y p e and t h e a d s o r p t i o n r e a c h e d a p l a t e a u v a l u e o f ~20 mg m . No d e s o r p t i o n was d e t e c t e d when t h e c e n t r i f u g e a s e d i m e n t was r e d i s p e r s e d i n t o water. The p l a t e a u a d s o r p t i o n v a l u e i s much higher than t h a t r e c e n t l y o b t a i n e d on p o l y s t y r e n e l a t e x p a r t i c l e s (~3 mg m " ) . However, i t s h o u l d be m e n t i o n e d t h a t t h e a d s o r p t i o n v a l u e s were b a s e d on t h e BET s u r f a c e a r e a o f t h e powder, w h i c h c o u l d be an u n d e r e s t i m a t e o f t h e t r u e a r e a , p a r t i c u l a r l y s i n c e t h e p o w d e r s were d i s p e r s e d u s i n g u l t r a s o n i c i r r a d i a t i o n w h i c h may r e s u l t i n some a t t r i t i o n p r o d u c i n g fine p a r t i c l e s which would i n c r e a s e the s u r f a c e area s i g n i f i c a n t l y . S i n c e t h e o b j e c t o f t h e a d s o r p t i o n e x p e r i m e n t was t o s t u d y i r r e v e r s i b i l i t y of a d s o r p t i o n , o b t a i n i n g the absolute f i g u r e f o r t h e a d s o r p t i o n was n o t c o n s i d e r e d i m p o r t a n t a n d no a t t e m p t was made t o m e a s u r e t h e s u r f a c e a r e a o f t h e p a r t i c l e s when d i s p e r s e d in solution. 2

Viscosity-volume

fraction

curves

Fig.2

shows t h e

viscosity

as

a

f u n c t i o n o f volume f r a c t i o n o f p a r t i c l e s . The r e s u l t s a r e t y p i c a l of those u s u a l l y obtained with concentrated d i s p e r s i o n s ( 6 ) , s h o w i n g a r a p i d i n c r e a s e i n v i s c o s i t y above a c r i t i c a l volume f r a c t i o n o f the d i s p e r s e d phase. When t h e v o l u m e f r a c t i o n reaches the s o - c a l l e d packing f r a c t i o n , φ (see D i s c u s s i o n S e c t i o n ) , the v i s c o s i t y reaches a very hign value. may be o b t a i n e d f r o m a p l o t o f V5HTr v e r s u s Φ 3 a n d e x t r a p o l a t i o n t o 1//Ti^=0. T h i s gave a φ v a l u e o f ~0.73 f o r the bare particles. W i t h an a v e r a g e p a r t i c l e s i z e o f 1μπι, t h e a c t u a l v o l u m e f r a c t i o n ( p a r t i c l e + a d s o r b e d l a y e r ) w i l l be o n l y s l i g h t l y l a r g e r than t h i s value. A s s u m i n g an a d s o r b e d l a y e r t h i c k n e s s o f 5nm ( w h i c h i s a r e a s o n a b l e e s t i m a t e f o r a l a y e r o f PEO with M=750), φ f o r p a r t i c l e and l a y e r i s -0.74. sp 3 ρ

I n f l u e n c e o f a d d i t i o n o f f r e e p o l y m e r on t h e r h e o l o g y o f t h e suspensions F i g . 3 shows t h e e f f e c t o f a d d i t i o n o f PEO (M =20,000, 35,000 and 90,000 r e s p e c t i v e l y ) on t h e e x t r a p o l a t e d y i e l d value o b t a i n e d from the shear s t r e s s - s h e a r r a t e curves u s i n g t h e Haake R o t o v i s k o . On t h e o t h e r h a n d , F i g u r e 4 shows t h e r e s u l t s o b t a i n e d u s i n g the Deer rheometer i n which was d i r e c t l y obtained. A l t h o u g h t h e t r e n d o b t a i n e d i s t h e same, t h e y i e l d v a l u e s o b t a i n e d u s i n g t h e Deer rheometer a r e s i g n i f i c a n t l y w




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HEATH ET AL.

Figure

3.


Effect

o f PEO

extrapolated

concentration y i e l d value

on

the

(using the

Haake

Rotovisko).


18


lower than those obtained u s i n g the e x t r a p o l a t i o n procedure. T h i s i s expected s i n c e the e x t r a p o l a t i o n procedure u s u a l l y overestimates the y i e l d v a l u e . The d a t a o f F i g s . 3 a n d 4 show a r a p i d i n c r e a s e i n y i e l d v a l u e a b o v e a c r i t i c a l PEO concentration, φ . T h i s c o n c e n t r a t i o n corresponds to the c r i t i c a l f l o c c u l a t i o n c o n c e n t r a t i o n of the f r e e polymer. However, s i n c e the r i s e i n d i d n o t o c c u r a t a s h a r p φ^ v a l u e , t h e l a t t e r was t a k e n as t h e i n t e r s e c t i o n p o i n t a t w h i c h t h e e x t r a p o l a t e d h o r i z o n t a l and v e r t i c a l l i n e s meet. T h i s gave v a l u e s o f φ of 0 . 0 2 ± 0 . 0 0 2 , 0.01±0.001 a n d 0.005+0.001 f o r PEO w i t h M of 20,000, 35,000 a n d 90,000 r e s p e c t i v e l y . M o r e o v e r , t h e φ v a l u e s o b t a i n e d f r o m t h e two s e t s o f r h e o l o g i c a l r e s u l t s were a l m o s t t h e same w i t h i n t h e e r r o r o f l o c a t i n g φ^. w


ρ

S e d i m e n t h e i g h t and r e d i s p e r s i o n As an i l l u s t r a t i o n t h e r e s u l t s o b t a i n e d u s i n g PEO 20,000 a r e shown i n F i g . 5. T h i s f i g u r e shows t h e v a r i a t i o n o f s e d i m e n t h e i g h t a n d number o f r e v o l u t i o n s r e q u i r e d f o r r e d i s p e r s i o n as a f u n c t i o n o f PEO concentration, e x p r e s s e d as volume f r a c t i o n o f p o l y m e r i n t h e c o n t i n u o u s p h a s e , φ . W i t h o u t a d d e d PEO, t h e s e d i m e n t h e i g h t was q u i t e h i g h and t h e w h o l e d i s p e r s i o n was t u r b i d ( i n d i c a t e d by l e t t e r Τ on t h e diagram). On s t a n d i n g t h i s d i s p e r s i o n s e t t l e d t o t h e b o t t o m o f t h e t u b e l e a v i n g a t u r b i d s u p e r n a t a n t and t h e s e d i m e n t formed c o u l d n o t be r e d i s p e r s e d a f t e r r o t a t i n g t h e t u b e f o r more t h a n 100 r e v o l u t i o n s i n d i c a t i n g f o r m a t i o n o f a h a r d c l a y a t t h e b o t t o m of the tube. On a d d i t i o n o f 0.5% PEO 20,000 ( φ = 0 . 0 0 7 ) , t h e d i s p e r s i o n a p p e a r e d t u r b i d and t h e e q u i l i b r i u m sediment h e i g h t was r e d u c e d . However, t h e s e d i m e n t c o u l d be r e d i s p e r s e d by a p p l i c a t i o n of 5 r o t a t i o n s t o the tube. F u r t h e r i n c r e a s e i n PEO c o n c e n t r a t i o n r e s u l t e d i n a c l e a r s u p e r n a t a n t l i q u i d and t h e s e d i m e n t h e i g h t r e a c h e d a minimum h e i g h t b e t w e e n 1 a n d 2% PEO (φρ b e t w e e n 0.015 a n d 0 . 0 2 7 ) , a b o v e w h i c h t h e r e was a continuous i n c r e a s e i n sediment h e i g h t with c l e a r s u p e m a t a n t s . A b o v e 1-2% PEO, t h e s e d i m e n t became more v i s c o u s a l t h o u g h no claying occurred. T h e s e v i s c o u s s e d i m e n t s were more d i f f i c u l t t o r e d i s p e r s e and t h e number o f r e v o l u t i o n s r e q u i r e d f o r r e d i s p e r s i o n i n c r e a s e d s h a r p l y a b o v e 3% PEO ( φ = 0 . 0 4 ) . Thus, t h e sediment h e i g h t e x p e r i m e n t s i n d i c a t e f l o c c u l a t i o n between 1 a n d 2% PEO ( φ b e t w e e n 0.015 and 0.027); the e x a c t f l o c c u l a t i o n c o n c e n t r a t i o n was d i f f i c u l t t o a s s i g n . Therefore, the r e d i s p e r s i o n e x p e r i m e n t s showed c l a y i n g i n t h e a b s e n c e o f PEO and a h i g h l y v i s c o u s s u s p e n s i o n a b o v e t h e f l o c c u l a t i o n p o i n t by t h e f r e e polymer. ρ

ρ

ρ




HEATH ET AL.

Figure

5.

Sediment h e i g h t f u n c t i o n o f PEO

and r e d i s p e r s i o n as a 20,000 c o n c e n t r a t i o n .


20


Discussion D i s p e r s i n g power o f t h e "comb" s t a b i l i s e r Both adsorption i s o t h e r m s a n d v i s c o s i t y - v o l u m e f r a c t i o n c u r v e s show c l e a r l y t h e a b i l i t y o f t h e "comb" s t a b i l i s e r t o p r o d u c e h i g h l y d e f l o c c u l a t e d suspensions. T h u s , t h e s t r o n g a n c h o r i n g p r o d u c e d by t h e b a c k b o n e o f t h e g r a f t c o p o l y m e r (no d e s o r p t i o n was d e t e c t e d when t h e s e d i m e n t was d i l u t e d w i t h w a t e r ) a n d t h e s t e r i c s t a b i l i s a t i o n due t o t h e PEO c h a i n s w h i c h a r e w e l l s o l v a t e d i n w a t e r , e n a b l e s one to produce suspensions approaching the t h e o r e t i c a l packing f r a c t i o n (0.74 f o r a m o n o d i s p e r s e h e x a g o n a l l y c l o s e - p a c k e d d i s p e r s i o n and h i g h e r v a l u e s f o r p o l y d i s p e r s e s y s t e m s ) . This i s shown by t h e η "-φ c u r v e ( F i g . 2 ) w h i c h , u s i n g t h e e x t r a p o l a t i o n procedure described i n the Results s e c t i o n , gives a packing fraction, Φ / o f 0.74. The i n t e r a c t i o n i n t h i s s t e r i c a l l y s t a b i l i s e d a i s p e r s i o n may be r e p r e s e n t e d by a h a r d s p h e r e m o d e l w i t h a h a r d s p h e r e r a d i u s o f (a+6) where 6 i s t h e a d s o r b e d l a y e r thickness. I f t h i s i s t h e c a s e , t h e n i t s h o u l d be p o s s i b l e t o f i t t h e η-φ c u r v e t o t h e D o u g h e r t y - K r i e g e r e q u a t i o n ( 6 , 7 ) .


Γ

Ό 8

n

r

« Π-(Φ /Φ 3

8

)]~

[

n

]

*s

P

(1)

Using the extrapolated φ v a l u e and t h e t h e o r e t i c a l v a l u e o f [η] o f 2.5, T) was c a l c u l a t e d a s a f u n c t i o n o f φ a n d t h e r e s u l t s a r e shown f o r c o m p a r i s o n i n F i g 2. The t h e o r e t i c a l - φ curve i s s i g n i f i c a n t l y lower than the e x p e r i m e n t a l curve. T h i s may be due t o t h e v a n d e r W a a l s a t t r a c t i o n a s t h e p a r t i c l e s are c l o s e together i n the very concentrated suspension. T h e r e f o r e , t h e h a r d s p h e r e m o d e l c a n n o t be a p p l i e d t o a v e r y c o n c e n t r a t e d d i s p e r s i o n w i t h o u t i n t r o d u c i n g a p e r t u r b a t i o n due t o t h e van der Waals a t t r a c t i o n . r

η

Γ

Interpretation of rheological results The t r e n d s i n t h e v a r i a t i o n of w i t h φ^ a r e s i m i l a r t o t h o s e o b t a i n e d r e c e n t l y (3) u s i n g a m o d e l p o l y s t y r e n e l a t e x d i s p e r s i o n . The φ + v a l u e s o b t a i n e d i n the p r e s e n t system are a l s o c l o s e t o t h o s e o b t a i n e d w i t h t h e m o d e l d i s p e r s i o n ( 0 . 0 1 7 , 0.008 a n d 0.005 f o r PEO 20,000, 35,000 a n d 90,000 r e s p e c t i v e l y ) . As m e n t i o n e d before the sharp i n c r e a s e i n above φ+ i n d i c a t e s t h a t a t t h e o n s e t o f f l o c c u l a t i o n t h e d i s p e r s i o n s show m a r k e d viscoelasticity. The f l o c c u l a t i o n o b t a i n e d a t φ ^ c o r r e s p o n d s t o t h e o n s e t o f t h e " s e m i d i l u t e " r e g i o n , φ^, i . e . , where t h e p o l y m e r c o i l s i n s o l u t i o n b e g i n t o a r r a n g e t h e m s e l v e s i n some


2.

HEATH ET AL.

close-packed The

a r r a y as

v a l u e o f φ*

polymer

i n bulk

21

Stabilization of Aqueous Suspensions a result

corresponding solution

c a n be

of excluded to the

volume

effects.

o v e r l a p of the

c a l c u l a t e d using the

free equation

(8)

(2) Φ

ρ

=

b* is t h e r a d i u s o f g y r a t i o n o f t h e p o l y m e r , b * i s a c o n s t a n t (5.63 f o r hexagonal c l o s e packing of the polymer), N i s Avogadro's number a n d p i s t h e d e n s i t y o f t h e p o l y m e r . The v a l u e o f ^ was c a l c u l a t e d f r o m t h e i n t r i n s i c v i s c o s i t y o f t h e c o r r e s p o n d i n g polymer s o l u t i o n u s i n g the Stockmayer-Fixman r e l a t i o n s h i p ( 9 ) . The v a l u e s f o r φ * d e r i v e d were f o u n d t o be 0.029, 0.02 a n d 0.01 f o r PEO w i t h M o f 20,000, 35,000 a n d 90,000 respectively. I t seems t h a t t h e φ* v a l u e s a r e a l w a y s h i g h e r than the φ+ v a l u e s .


A

2

2

P

+ W i t h f u r t h e r i n c r e a s e i n φ^ a b o v e φ^, t h e y i e l d v a l u e c o n t i n u e s t o i n c r e a s e i n d i c a t i n g an i n c r e a s e i n t h e e x t e n t o f flocculation. S e v e r a l mechanisms were p r o p o s e d t o e x p l a i n t h e f l o c c u l a t i o n i n d u c e d by t h e a d d i t i o n o f f r e e (non-adsorbing polymer). The f i r s t e x p l a n a t i o n was g i v e n by A s a k u r a a n d Oosawa (10). When two p a r t i c l e s a p p r o a c h e a c h o t h e r w i t h i n a d i s t a n c e of s e p a r a t i o n t h a t i s s m a l l e r than the diameter of the f r e e polymer c o i l , e x c l u s i o n o f the polymer from the i n t e r s t i c e s between t h e p a r t i c l e s t a k e s p l a c e l e a d i n g t o t h e f o r m a t i o n of a polymer f r e e zone. T h i s p r o d u c e s an a t t r a c t i v e f o r c e a s s o c i a t e d w i t h a lower o s m o t i c p r e s s u r e i n t h e r e g i o n between t h e particles. Thus, t h e f l o c c u l a t i o n r e s u l t s from d e p l e t i o n o f the f r e e polymer from t h e i n t e r s t i c e s between t h e p a r t i c l e s and hence i s u s u a l l y r e f e r r e d t o as d e p l e t i o n f l o c c u l a t i o n . The t h e o r y o f A s a k u r a a n d Oosawa (10) h a s b e e n e i t h e r e x t e n d e d and/or m o d i f i e d by v a r i o u s i n v e s t i g a t o r s . F o r example, V r i j (11) h a s d e v e l o p e d a t h e o r y f o r d é s t a b i l i s a t i o n b a s e d on t h e e x p u l s i o n of t h e polymer from t h e i n t e r s t i t i a l spaces between a p p r o a c h i n g p a r t i c l e s b a s e d on "volume r e s t r i c t i o n " a s w e l l a s " o s m o t i c " c o n s i d e r a t i o n s . V i n c e n t e t a l (8) h a v e s p e c i f i c a l l y c o n s i d e r e d t h e c a s e where t h e p a r t i c l e s c a r r y an a d s o r b e d polymer, l a y e r ; t h e y c a l c u l a t e d t h e n e t f r e e e n e r g y c h a n g e i n v o l v e d when t h e f r e e polymer c o i l s a r e d i s p l a c e d from the r e g i o n between t h e p a r t i c l e s into bulk s o l u t i o n . In t h e i r t h e o r y , o n l y m i x i n g terms a r e c o n s i d e r e d ; the n e t f r e e energy i n v o l v e d i n the exchange p r o c e s s amounts i n e f f e c t t o t h e f r e e e n e r g y o f m i x i n g o f t h e


22



a d s o r b e d l a y e r s ( a t a g i v e n s e p a r a t i o n ) minus the f r e e energy g a i n e d by t h e d e m i x i n g o f t h e ( d i s p l a c e d ) p o l y m e r c o i l s w i t h t h e a d s o r b e d l a y e r s . F e i g i n and N a p p e r (12) h a v e d e v e l o p e d a t h e o r y b a s e d upon r o t a t i o n a l i s o m e r i c s t a t e Monte C a r l o p r o c e d u r e s f o r c a l c u l a t i n g segment c o n c e n t r a t i o n p r o f i l e s o f m a c r o m o l e c u l e s i n t h e r e g i o n b e t w e e n two p a r t i c l e s . They then c a l c u l a t e d the change i n the o v e r a l l f r e e energy of m i x i n g w i t h d i s t a n c e o f s e p a r a t i o n between the p l a t e s u s i n g F l o r y - H u g g i n s t h e o r y o f polymer s o l u t i o n s . The above t h e o r i e s may be u s e d f o r i n t e r p r e t a t i o n o f t h e r h e o l o g i c a l r e s u l t s , i n p a r t i c u l a r the i n c r e a s e i n the B i n g h a m y i e l d v a l u e , τ ^ , w i t h i n c r e a s e i n φ^. As m e n t i o n e d b e f o r e (3) c a n be a n a l y s e d i n t e r m s o f i n t e r p a r t i , c l e interactions. T h i s may be e q u a t e d t o t h e amount o f e n e r g y n e e d e d t o t o t a l l y s e p a r a t e the f l o e s i n t o s i n g l e u n i t s (13,14), i e . To = Ν ρ

E

^ sep

(3)

e

where Ν i s t h e t o t a l number o f c o n t a c t s b e t w e e n p a r t i c l e s i n f l o e s and E i s the energy r e q u i r e d t o break each c o n t a c t . The t o t a l number o f c o n t a c t s , N, may be r e l a t e d t o t h e p a r t i c l e volume f r a c t i o n , φ and t h e a v e r a g e number o f c o n t a c t s p e r p a r t i c l e , n, by ( 1 3 ) , g

e

3

3 ώ Ν =

η

(—±1

1/2

4 π a

)

(4)

3

where a i s t h e p a r t i c l e

=

3

6

n E

*s

radius.

Combining equations

sep

(3)

and

(4),

( 5 )

3

8π

3

E q u a t i o n (5) shows t h a t energy of s e p a r a t i o n , E

g e

p,

i s d i r e c t l y p r o p o r t i o n a l to the needed t o separate the f l o e s .

E

g

may be e q u a t e d t o t h e f r e e e n e r g y minimum, G , i n the f r e e e n e r g y - d i s t a n c e c u r v e i n t h e p r e s e n c e o f f r e e p o l y m e r . j(i± may be e s t i m a t e d u s i n g t h e t h e o r y o f V i n c e n t e t a l ( 8 ) ; t h e d e t a i l s of the c a l c u l a t i o n s are given i n the Appendix. Fig. 6 shows t h e v a r i a t i o n o f G . ( i n kT u n i t s ) w i t h φ f o r the mm ρ t h r e e PEO m o l e c u l a r w e i g h t s u s e d . It i s clear that G.^ m i n

G

n

m

Λ

3

increases with G

min

i

with

φρ n

c

r

e

increase

nun in φ

(see F i g s . 3 and a

s

e

s

with

, thus e x p l a i n i n g the

4?.

M o r e o v e r , a t any

i n c r e a s e of M , w

increase

given

thus e x p l a i n i n g

φ , ρ

the


in

2.

increase

in

with

concentration. In p r i n c i p l e ,


G

v

a

l

u

e

23


HEATH ET AL.

increase of

at a given free

i t i s p o s s i b l e t o c a l c u l a t e τg

polymer

from

the

s

min g i v e n i n F i g . 6, u s i n g e q u a t i o n ( 5 ) . I f the f l o e s a r e assumed t o be c l o s e - p a c k e d s t r u c t u r e s , t h e n η must be between 8 ( r a n d o m - c l o s e p a c k i n g ) a n d 12 ( h e x a g o n a l c l o s e - p a c k i n g ) . However, e q u a t i o n (5) i s b a s e d on a m o n o d i s p e r s e s u s p e n s i o n . With a suspension c o n t a i n i n g a d i s t r i b u t i o n of p a r t i c l e s i z e s , one c a n n o t r e p l a c e a by t h e a v e r a g e p a r t i c l e s i z e , s i n c e t h e number o f c o n t a c t p o i n t s Ν i n a f l o e i s g r e a t e r t h a n t h a t c a l c u l a t e d u s i n g an a v e r a g e p a r t i c l e s i z e . With a p o l y d i s p e r s e s u s p e n s i o n i t i s d i f f i c u l t t o c a l c u l a t e t h e number o f c o n t a c t s w i t h o u t knowledge o f the e x a c t p a r t i c l e s i z e d i s t r i b u t i o n . Since t h i s was n o t a v a i l a b l e i n t h e p r e s e n t s y s t e m , no a t t e m p t was made to calculate ( a n d h e n c e compare i t w i t h t h e e x p e r i m e n t a l value). However, t h e t r e n d i n ρ"Φρ c o r r e l a t e s very w e l l with the i ^ p curves thus c o n f i r m i n g the v a l i d i t y of the model u s e d t o e s t i m a t e G . . mi η τ

G

m

n

S e d i m e n t h e i g h t and r e d i s p e r s i o n The r e s u l t s o f t h e s e d i m e n t h e i g h t a n d r e d i s p e r s i o n e x p e r i m e n t s ( F i g . 5) a r e c o n s i s t e n t w i t h t h e m o d e l o f weak f l o c c u l a t i o n and t h e r h e o l o g i c a l d a t a d e s c r i b e d above. I n t h e a b s e n c e o f any a d d e d f r e e p o l y m e r , t h e d i s p e r s i o n i s c o l l o i d a l l y s t a b l e a n d any s e p a r a t i o n as a r e s u l t o f sedimentation leaves a t u r b i d supernatant of c o l l o i d a l l y s t a b l e particles. The s e d i m e n t f o r m e d i s h a r d ( i n t e c h n i c a l t e r m s a " c l a y " ) a s a r e s u l t o f t h e d e n s e p a c k i n g o f t h e p a r t i c l e s . The l a t t e r w h i c h s e d i m e n t u n d e r g r a v i t y a r e a b l e t o move p a s t e a c h o t h e r (as a r e s u l t o f t h e s t r o n g r e p u l s i v e f o r c e between t h e p a r t i c l e s ) forming a close-packed sediment r e q u i r i n g a l a r g e number o f r e v o l u t i o n s f o r r e s u s p e n s i o n . On a d d i t i o n o f f r e e p o l y m e r , t h e s e d i m e n t h e i g h t i n i t i a l l y d e c r e a s e s and t h e n r e a c h e s a minimum. T h i s i s a c c o m p a n i e d by t h e f o r m a t i o n o f c l e a r supernatant l i q u i d i n d i c a t i v e of p a r t i c l e f l o c c u l a t i o n . Under t h e s e c o n d i t i o n s , t h e weakly f l o c c u l a t e d sediment i s easy t o redisperse. However, w i t h f u r t h e r i n c r e a s e i n f r e e p o l y m e r c o n c e n t r a t i o n , t h e e x t e n t of f l o c c u l a t i o n i n c r e a s e s and t h e y i e l d v a l u e i n c r e a s e s . T h i s i s a c c o m p a n i e d by an i n c r e a s e i n s e d i m e n t h e i g h t a n d t h e s e d i m e n t becomes more d i f f i c u l t t o r e d i s p e r s e as a r e s u l t of the i n c r e a s e i n v i s c o s i t y . Thus, the sediment h e i g h t and r e d i s p e r s i o n e x p e r i m e n t s a r e c o n s i s t e n t w i t h the r h e o l o g i c a l results. Conclusions H i g h l y d e f l o c c u l a t e d c o n c e n t r a t e d s u s p e n s i o n s c a n be o b t a i n e d u s i n g b l o c k o r g r a f t c o p o l y m e r s c o n s i s t i n g o f an i n s o l u b l e component h a v i n g a h i g h a f f i n i t y t o t h e p a r t i c l e s u r f a c e a n d s o l u b l e c h a i n s ( s t r o n g l y s o l v a t e d by t h e medium) w h i c h p r o v i d e the s t a b i l i s i n g moiety. These s t e r i c a l l y s t a b i l i s e d suspensions




24


2.

HEATH ET

25


AL.

c a n be w e a k l y f l o c c u l a t e d by t h e a d d i t i o n o f f r e e n o n - a d s o r b i n g p o l y m e r , above a c r i t i c a l c o n c e n t r a t i o n w h i c h d e p e n d s on t h e m o l e c u l a r weight of the f r e e polymer. These weakly f l o c c u l a t e d s t r u c t u r e s a r e e a s y t o r e d i s p e r s e and, t h e r e f o r e , t h e phenomenon may be u s e d t o p r e v e n t c l a y i n g .


Acknowledgements M o s t o f t h e e x p e r i m e n t work d e s c r i b e d i n t h i s p a p e r has b e e n c a r r i e d o u t by Mr R R a j a r a m ( B r u n e i U n i v e r s i t y ) d u r i n g a s i x month i n d u s t r i a l t r a i n i n g p e r i o d a t J e a l o t t ' s H i l l . The a u t h o r s a r e a l s o i n d e b t e d t o Mr S D o u g l a s f o r c a r r y i n g o u t t h e a d s o r p t i o n isotherms. We a r e g r a t e f u l t o Mr F W a i t e o f I C I P a i n t s D i v i s i o n f o r v a l u a b l e s u g g e s t i o n s and s t i m u l a t i n g d i s c u s s i o n s d u r i n g t h e course of t h i s study. We a r e a l s o g r a t e f u l t o Dr Β V i n c e n t f o r m a k i n g v a l u a b l e comments. Appendix Basic equations f o r c a l c u l a t i o n of f r e e energy-distance curves V i n c e n t e t a l (8) showed t h a t t h e n e t f r e e e n e r g y b a l a n c e i n v o l v e d i n d i s p l a c i n g c o i l s i n t o b u l k s o l u t i o n on o v e r l a p p i n g t h e s h e a t h on t h e i r a p p r o a c h i n g p a r t i c l e s may be a p p r o x i m a t e d by,

G

m

l

G

+

mix

s

1/2l J

A

2 (

t h e Hamaker c o n s t a n t s

of the pure

i

x

)

polymer

respectively.

f o r the

e x t e n s i o n of F l o r y

calculation 1

s theory

P

of G ? , V i n c e n t x

(17)

f o r two

e t a l (8) u s e d

interacting

chains

an

in


solution, i e .

+ 1

2

[2 < s >

3 / 2

V

iSl

3 q

(1/3

- X ) 2

exp

- |î

(x)

where r i s t h e c r o s s - s e c t i o n a l a r e a o f t h e f r e e p o l y m e r c o i l a n d ν i s t h e segment d e n s i t y a t t h e c e n t r e o f t h e p o l y m e r c o i l ,

Μ

Γ

v

L

3 2

s 2

< >

Π3/2 1

(xi)

-

The G - d i s t a n c e c u r v e s were e s t a b l i s h e d u s i n g t h e a b o v e e q u a t i o n s . The v a l u e s o f t h e v a r i o u s p a r a m e t e r s u s e d i n t h e s e t o t

β

&

c a l c u l a t i o n s a r e as f o l l o w s : a 500nm/S = 5.0nm, φ = 0.15, A / ) a r e 5.52, 7.59 a n d 12.90 nm f o r PEO w i t h M o f 20,000, 35,000 a n d 90,000 r e s p e c t i v e l y . From t h e s e e n e r g y - d i s t a n c e c u r v e s , t h e values of G . ( = E^) were e s t a b l i s h e d , mm sep ρ

2

1

2

S

2

m

Literature Cited 1. 2. 3. 4. 5.

Tadros, Th. F., Adv. Colloid Interface Sci. 12, 141 (1980). Tadros, Th. F., in "The Effects of Polymers on Dispersion Properties" Editor, Tadros, Th. F. Academic Press, London (1982) pp. 1-39. Heath, D. and Tadros, Th. F., Disc. Faraday Soc. (1983) in press. Baleaux, B. C. R. Hebd. Seances Acad. Sci. Ser C, 274, 1617 (1975). Tadros Th. F. and Vincent, B. J. Phys. Chem., 80, 1575 (1980).


A D V A N C E S IN PESTICIDE F O R M U L A T I O N T E C H N O L O G Y

28

6.

18.

T. J. Dougherty, Ph. D. Thesis, Case Institute of Technology (1959). Krieger, I.M.Advan. Colloid Interface Sci., 3, 111 (1972). Vincent, B. Luckham P.F. and Waite, F. J. Colloid Interface Sci., 73, 508 (1980). Stockmayer W.H. and Fixman M., J. Polymer Sci. C,1,137 (1963). Asakura S. and Oosawa F., J. Chem. Phys., 22, 1255 (1954). Vrij Α., Pure Appl. Chem., 48, 471 (1976). Feigin R.I. and Napper D.H., J. Colloid Interface Sci., 74, 567 (1980); 75, 525.(1980); D.H. Napper in "The Effect of Polymers on Dispersion Stability, ed. Th.F.Tadros (Academic Press, London, (1982) pp. 199206. Luckham P.F., Vincent B. and Tadros Th.F., Colloids and Surfaces, 6, 101 (1983). Gillespie T., J. Colloid Sci., 15, 219 (1960). Smitham J.B., Evans R. and Napper D.H., J. Chem. Soc. Faraday Trans 1, 72, 2425 (1976). Vincent B., J. Colloid Interface Sci., 42, 270 (1977). Flory P.J., "Principles of Polymer Chemistry" Cornell University Press, (1953),p. 535. Greig, B. and Tadros, Th.F., to be published.

RECEIVED

February 9,

7. 8. 9. 10.


11. 12.

13. 14. 15. 16. 17.

1984


Stabilization of Aqueous Pesticidal Suspensions by Graft Copolymers

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