Pesticide Formulations - American Chemical Society

Chapter 16

Mechanism of Action of Hydroxyethylcellulose as a Structure Agent for Suspension Concentrate Formulations David J. Wedlock

Pesticide Formulations Downloaded from pubs.acs.org by CORNELL UNIV on 10/11/16. For personal use only.

Shell Research, Sittingbourne Research Centre, Sittingbourne, Kent, ME9 8A6, England The suspension concentrate (SC) formulation f o r p e s t i c i d a l active ingredients is conventionally prepared by a m i l l i n g or comminution process using a wetting agent and/or dispersant to e f f e c t production of a c o l l o i d a l l y stable suspension of an active ingredient i n an aqueous continuous phase. This suspension i s a thermodynamically unstable system, and will undergo spontaneous sedimentation of the dispersed phase. The sediment formed i s generally d i l a t a n t i n rheological terms and thus will not r e a d i l y redisperse to give a homogeneous suspension. Structure agents, usually water soluble polymers, are added to such suspensions, i n order to i n h i b i t or slow down the sedimentation rate. The mechanism by which these structure agents function i s often poorly understood and thus we have set out to determine the mechanism of action of one s p e c i f i c structuring agent, hydroxyethyl-cellulose (HEC), as part of a wider programme of suspension characterisation.

The formulation of plant protection agents i n the form of suspensions has many advantages, not l e a s t the f a c t that one can avoid the use of organic solvents i n the formulation. Often b i o l o g i c a l a c t i v i t y can be maintained at l e v e l s comparable with emulsifiable systems with the judicious use of formulation o i l s and adjuvants. The main problems associated with suspensions i s that while they are r e l a t i v e l y easy to produce as c o l l o i d a l l y stable systems, they have an inherent tendency to become heterogeneous on storage, with the active ingredient sedimenting under the influence of gravity to y i e l d what are referred to as d i l a t a n t clay sediments.(1) The rheological properties of these "clays" means that they are not r e a d i l y redispersed by inversions since they tend to thicken, or the v i s c o s i t y increases with increasing shear rate. The sediment may pack to the close packed l i m i t i f sedimentation i s uncontrolled.

0097-6156/88/0371-O190$06.00/0 ° 1988 American Chemical Society


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Hydroxyethylcellulose as a Structure Agent

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I t i s common practice to add what are usually referred to as structure agents to the formulation i n order to retard the sedimentation rate. These are either water soluble polymers or f l o c c u l a t e d clays such as bentonite.(2) The water soluble polymers usually function by inducing a controlled l e v e l of f l o c c u l a t i o n between the suspended p a r t i c l e s of the system. The suspended p a r t i c l e s would be regarded as residing i n a minimum of the p a r t i c l e p a i r i n t e r a c t i o n p o t e n t i a l curve.(3) There are at l e a s t two p r i n c i p a l mechanisms proposed by which a water soluble polymer may induce f l o c c u l a t i o n of suspended p a r t i c l e s , and hence retard sedimentation and reduce the tendency to formation o f d i l a t a n t clays. One i s the bridging mechanism by which an adsorbing polymer may simultaneously adsorb to more than one p a r t i c l e , ( 3 ) leading to a network-like formation i n the suspension. This mechanism by implication requires that the polymer being used adsorbs strongly.(3) I t i s usually found that i n such systems the optimum conditions for bridging f l o c c u l a t i o n are when the polymer i s adsorbed to less than complete surface coverage of the p a r t i c l e by the polymer. Complete surface coverage i s usually ascertained by examination of the adsorption isotherm, where the l e v e l of polymer addition required to reach the plateau i n the adsorption isotherm often corresponds to complete surface coverage. Lesser additions of polymer may r e s u l t i n bridging and observable f l o c c u l a t i o n of p a r t i c l e s , although not invariably. The a l t e r n a t i v e mechanism of f l o c c u l a t i o n i s by the depletion mechanism, where a non-adsorbing polymer produces a depletion layer (4) or region o f negative adsorption around the p a r t i c l e , which may be shown to lead to a t t r a c t i v e interactions between p a r t i c l e s , r e s u l t i n g from a t t r a c t i v e osmotic forces due to excluded polymer. The f l o c c u l a t i o n of the p a r t i c l e s may be monitored by a number of techniques, such as sediment volume measurements, (3) t u r b i d i t y f l u c t u a t i o n , (5) or indicated by dispersed phase volume f r a c t i o n measurements. The l a t t e r technique i s a new experimental technique involving ultrasound v e l o c i t y measurements c a r r i e d out i n a v e r t i c a l scanning manner, (6) further developed i n these laboratories f o r applications with o p t i c a l l y opaque concentrated suspensions. Our i n i t i a l findings show that the technique can be used to quantify the state o f f l o c c u l a t i o n of suspended p a r t i c l e s i n terms of a volume bulk modulus using the measured volume f r a c t i o n at various v e r t i c a l positions i n a suspension. (7) The prime advantage i s that the development of the structure can be observed because the non-intrusive nature of the technique allows repeated scans of a given suspension, over a period of days, weeks or even months. Details of the adsorbed state of the structure agent, hydroxyethylcellulose (HEC), has been discerned using an EPR technique, where a spin l a b e l l e d derivative o f the polymer was produced using a covalently attached n i t r o x y l spin l a b e l (to a very low degree of l a b e l l i n g ) . ( 8 ) The r o t a t i o n a l c o r r e l a t i o n time of the spin l a b e l i s highly environment dependent, and thus to a f i r s t approximation we can determine whether the l a b e l and hence the polymer segment associated with i t i s i n a loop or t a i l configuration (mobile) or i n a t r a i n - l i k e or f l a t configuration (immobile).(9) This information correlates with the adsorption

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PESTICIDE FORMULATIONS: INNOVATIONS AND DEVELOPMENTS

isotherms and may suspension.

indicate the state of f l o c c u l a t i o n of the


Materials Suspension Preparation. The suspensions used were produced from technical 2-[[4-chloro-6-(ethylamino)-1,3,5-triazin-2-yl]amino]-2methylpropanenitrile, (cyanazine), using a proprietary polycarboxylate/polyisobutylene block copolymer as dispersant. The dispersions were 46% v/v when m i l l e d i n a Mini 250 Motor m i l l (Eiger Engineering Ltd, Warrington) with 2 mm glass media and a dispersant l e v e l of 17 g/kg was used. Product was m i l l e d to a p a r t i c l e sizes of 1.2 to 2.1 pa VMD (volume mean diameter), determined by Coulter counter (model TA 2), using cyanazine saturated e l e c t r o l y t e . Suspensions were d i l u t e d with d i s t i l l e d water. Natrosol JXR 250 (Hercules Ltd), an HEC of degree of substitution, 2.5 hydroxyethyl groups per glucose residue and Mw 1.0x10 was used to structure suspensions at the indicated l e v e l s and added to the suspensions post-milling, i n the form of a pre-dissolved concentrate. Preparation of Spin Labelled Polymer. Preparation of the spin l a b e l l e d HEC was performed using a v a r i a t i o n of the technique of Cafe et al,(8,9) o r i g i n a l l y developed f o r l a b e l l i n g of Na-carboxymethylcellulose. The spin l a b e l used was 2,2,6,6 tetramethyl-piperidino-oxyl (TEMPO). Cyanogen bromide was used to activate the HEC, but the a c t i v a t i o n reaction was c a r r i e d out f o r only 1 hour, compared to the 4 hours recommended f o r Na-CMC, and the pH of the reaction medium was maintained at a constant value of 10, f o r both the a c t i v a t i o n stage and the l i n k i n g stage. The higher values recommended f o r the reaction with Na-CMC l e d to unacceptably low values f o r the degree of l a b e l l i n g . Care was taken to ensure that the HEC d i d not c r o s s - l i n k during the a c t i v a t i o n stage by not exceeding 5g/l concentration of the HEC i n the reaction medium, and t h i s was further checked by measuring the i n t r i n s i c v i s c o s i t y of the HEC i n water, both before and a f t e r the l a b e l l i n g process, where i t was established that the increase i n i n t r i n s i c v i s c o s i t y was n e g l i g i b l e and within the experimental error of the technique. The samples of spin l a b e l l e d polymer were freed of excess non-covalently linked spin l a b e l by exhaustive d i a l y s i s of the samples against d i s t i l l e d water and checking the dialysate f o r the presence of an EPR s i g n a l . EPR spectra were recorded on a Brucker ER 200 spectrometer. Methods Adsorption Isotherms. Suspensions of c o l l o i d a l l y stable unstructured cyanazine p a r t i c l e s were prepared at the stated volume f r a c t i o n s , using a concentrated (46% v/v) stock suspension. Appropriate amounts of stock aqueous solutions of Natrosol JXR 250 were added to the concentrated suspensions with water. Samples were e q u i l i b r a t e d by tumbling overnight, followed by centrifuging at 12000 rpm. Supernatants were assayed f o r t h e i r hydroxyethylc e l l u l o s e content using the phenol-sulphuric a c i d reaction.(10)

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F l o c c u l a t i o n Monitoring


Sediment Volume Determination. These were performed on 7% v/v cyanazine suspensions contained i n conical based measuring cylinders, with various doses o f HEC added. The samples reached a reasonable degree o f consolidation within two weeks, a f t e r which the sediment volume was read. Turbidity Fluctuation. The t u r b i d i t y f l u c t u a t i o n technique has been described i n d e t a i l by other workers.(5) I t was successfully applied here to the same 7% v/v suspensions used i n the sedimentation volume measurements, a f t e r gentle inversions of the suspensions. The equipment used was a PDA 2000 photometric dispersion analyser (Rank Bros, Bottisham, Cambs, U.K.), and the output was monitored v i a the r a t i o channel. EPR Studies of Adsorbed Polymer. Samples f o r adsorbed polymer configuration studies were prepared by addition o f appropriate amounts o f spin l a b e l l e d Natrosol JXR 250 i n a pre-dissolved state,to c o l l o i d a l l y stable suspensions o f cyanazine, d i l u t e d to the appropriate volume f r a c t i o n from a concentrated stock suspension. The suspensions were allowed to e q u i l i b r a t e with tumbling f o r 18 hours,which preliminary experiments had shown was s u f f i c i e n t time to reach an equilibrium d i s t r i b u t i o n of polymer .Suspensions were then centrifuged at 12000 rpm,and the supernatant removed,followed by redispersal i n cyanazine saturated water.This procedure was repeated approximately 5 times or u n t i l no detectable spin l a b e l l e d polymer was found i n the supernatant.The polymer coated cyanazine p a r t i c l e s were then redispersed i n the minimum amount o f water and the EPR spectrum taken,using an aqueous solution type EPR c e l l to minimise d i e l e c t r i c losses. Ultrasound V e l o c i t y Studies of Suspensions. The technique has been described i n some d e t a i l previously, i n i t s a p p l i c a t i o n f o r studying creaming phenomena i n emulsion formulations. (6) I t has been further developed i n these laboratories and applied to the study o f sedimentation i n suspensions. In essence we measure the time o f f l i g h t of an i n d i v i d u a l , short duration (single cycle of RF) ultrasound pulse of 6 M Hz frequency, through a c e l l of 32 mm i n t e r n a l path length. The i n t e r n a l path length o f the c e l l i s measured a t 1 mm v e r t i c a l i n t e r v a l s , using two c a l i b r a t i o n l i q u i d s of known ultrasound v e l o c i t y , water and n-heptane a t 20 C. A l l measurements were c a r r i e d out at 20°C, i n a waterbath controlled to + 0.01 C. A schematic diagram of the e s s e n t i a l e l e c t r o n i c components i s shown i n reference 6. In addition to t h i s there i s a microcomputer controlled chassis holding the transducers which i s capable of logging the transducer height and time o f f l i g h t of the u l t r a s o n i c pulses, from which the v e l o c i t y (and hence volume f r a c t i o n ) i s calculable. A parabolic relationship between ultrasound v e l o c i t y through the suspension and dispersed phase volume f r a c t i o n was shown to hold over a volume f r a c t i o n range of at l e a s t 0 to 0.4, the so-called Urick relationship,(11) and i t was considered reasonable to apply the relationship to volume fractions up to the close-packed l i m i t . Volume fraction/height

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p r o f i l e s were obtained, with a p r e c i s i o n of 1 mm of v e r t i c a l displacement. This i s obtained using 10 mm diameter immersible transducers, since i n the f a r f i e l d the ultrasound i n t e n s i t y i s concentrated within a c y l i n d r i c a l element approximately 1 mm from the axis of the c y l i n d r i c a l transducer. Outside of t h i s element the ultrasound i n t e n s i t y drops by about -20 dB.(12)


Results and Discussion The preparation of the spin l a b e l l e d HEC derivative was determined to have l a b e l l e d the polymer to less than one sugar residue i n approximately 40, and thus i t was concluded that the spin l a b e l l e d derivative had s i m i l a r physico-chemical properties to the unlabelled analogue. The i n t r i n s i c v i s c o s i t y value of the derivative s i m i l a r l y indicated that there was no s i g n i f i c a n t c r o s s - l i n k i n g i n the a c t i v a t i o n process, see Table I.

Table I.

Type

I n t r i n s i c V i s c o s i t y and Molecular Weight Data for Natrosols I n t r i n s i c V i s c o s i t y (ml/g)

JXR JXR *

(labelled)

[YI] - 9 . 5 x l 0 " M 5

Molecular Weight*

216

l.OlxlO

5

226

1.07xlQ

5

0,85

F i g 1 shows the adsorption isotherms f o r the unlabelled polymer adsorbing onto c o l l o i d a l l y stable cyanazine p a r t i c l e s of 1.2 urn volume mean diameter, where the adsorption experiments were performed on suspensions of 0.07 and 0.40 dispersed phase volume f r a c t i o n suspensions of cyanazine respectively. I t may be seen that when the adsorption experiment i s c a r r i e d out at the lower volume fraction,4>-0.07, the isotherm i s apparently much more l i k e the c l a s s i c high a f f i n i t y type of isotherm, where the change over from the high a f f i n i t y region to the plateau l e v e l of polymer adsorption i s over a much more c r i t i c a l concentration range. For the adsorption experiment c a r r i e d out at the higher volume f r a c t i o n of p a r t i c l e s , the isotherm i s observed to curve gently away from the high a f f i n i t y region, and i n the concentration range of HEC investigated, not to reach the plateau l e v e l of polymer adsorption reached f o r the low volume f r a c t i o n experiment. This i s due to p o l y d i s p e r s i t y of the adsorbing HEC influencing the adsorption isotherm. (13,14) This e f f e c t has been noted by previous workers, and a d e f i n i t i v e t h e o r e t i c a l treatment has been given i n which i t has been shown that f o r a given p a r t i c l e surface area to volume of continuous phase r a t i o , i . e . a "concentration" of surface, there w i l l be only one molecular weight of polymer i n the continuous molecular weight d i s t r i b u t i o n which i s simultaneously adsorbed on the p a r t i c l e and free i n the continuous phase. Thus the polymer


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O


5

10

195

15

Equilibrium concentration of NATROSOL mg/ml

F i g 1. Natrosol JXR 250/cyanazine adsorption isotherms at volume fractions o f cyanazine 0.07 and 0.4.


PESTICIDE FORMULATIONS: INNOVATIONS AND DEVELOPMENTS e f f e c t i v e l y i s fractionated, the higher molecular weight fractions being adsorbed on to the p a r t i c l e and the lower molecular weight fractions remaining soluble i n the continuous phase. The e f f e c t of t h i s on the adsorption isotherms as a function of volume f r a c t i o n i s that at the higher volume fractions of p a r t i c l e s , a much larger dose of HEC i s required to achieve the same degree of surface coverage of the p a r t i c l e s by the HEC as i s obtained with the lower volume f r a c t i o n suspensions. This i s complemented by the observations of the adsorbed polymer configuration. I t may be seen i n F i g 2, where the EPR spectra of the surface adsorbed l a b e l l e d HEC f o r suspensions made at the lower p a r t i c l e volume f r a c t i o n of 0.07 are shown f o r d i f f e r e n t points on the adsorption isotherm. The EPR spectra f o r the points i n the high a f f i n i t y region, i . e . areas of low surface coverage, show the broad structureless features to be expected of a f a i r l y immobile spin probe adsorbed with polymer residues l y i n g i n t r a i n s on the p a r t i c l e surface, see F i g 3. I t i s worth comparing these spectra with the spectrum of the completely free polymer i n aqueous solution,see F i g 4. As the degree of surface coverage of the cyanazine p a r t i c l e by HEC i s increased, at higher polymer doses, the spectra of the surface adsorbed HEC takes on a much more mobile character, and this can be r a t i o n a l i s e d as the polymer having to take on a much more loopy configuration with a high proportion of the residues i n the form of loops and dangling t a i l s as the available surface for adsorption becomes much more r e s t r i c t e d , see F i g 3. The spectra i n F i g 2 were a l l obtained from 0.07 volume f r a c t i o n suspensions. In F i g 5, the spectra for d i f f e r e n t HEC doses equilibrated with 0.40 volume f r a c t i o n suspensions are shown. Even at the highest polymer dose the spectra have the c h a r a c t e r i s t i c s of an immobile spectrum. At no polymer dose was a mobile spectrum obtained f o r surface adsorbed HEC i n the 0.4 volume f r a c t i o n suspensions. This i s consistent with the adsorption isotherms, where at the comparable l e v e l s of surface coverage i n the low volume f r a c t i o n suspensions, only immobile behaviour was observed. In F i g 6, i t may be seen that the e f f e c t of addition of higher l e v e l s of the dispersant, with the spin l a b e l l e d HEC, does not appear to i n h i b i t adsorption of the HEC, although i t does influence the adsorbed HEC configuration. As the l e v e l of dispersant i s increased, the adsorbed HEC takes on a more mobile conformation, presumably because i t competes f o r the available space, forcing the HEC to loop more into the continuous phase. At the highest l e v e l of addition of dispersant, the adsorbed HEC i s remarkably mobile, having an EPR spectrum with almost the same l i n e width c h a r a c t e r i s t i c s as the free polymer. The observation of surface adsorbed spin l a b e l l e d HEC that cannot be removed by washing unequivocally shows that the HEC does not f l o c c u l a t e the suspension by a depletion mechanism. F l o c c u l a t i o n Studies. In F i g 7, the r e s u l t s of the sediment volume and t u r b i d i t y f l u c t u a t i o n studies are presented. The studies are r e s t r i c t e d to the 0.07 volume f r a c t i o n suspensions f o r p r a c t i c a l reasons. They show that the c o l l o i d a l s t a b i l i t y of the cyanazine p a r t i c l e s are sensitive to the concentration of added HEC. There


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ffydroxyethylcelluhseas a Structure Agent 197

F i g 4.

Electron spin resonance spectrum of free spin l a b e l l e d Natrosol JXR 250 i n water.



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199

Centrefield 3470 Gauss Sweep width 100 Gauss Room temperature

F i g 5.

Electron spin resonance spectra of surface adsorbed polymer as a function of polymer dose, volume f r a c t i o n of cyanazine - 0.4.



200

F i g 6.

Electron spin resonance spectrum of Natrosol JXR 250, as a function of dispersant dose.

F i g 7. F l o c c u l a t i o n monitoring of low volume f r a c t i o n cyanazine suspensions, using photometric dispersion analysis.



202


i s a polymer dose corresponding to less than complete surface coverage at which the cyanazine p a r t i c l e s f l o c c u l a t e . This i s quite c l e a r l y seen by both f l o c c u l a t i o n monitoring techniques. The sediment volume measurements show that at less than surface coverage a low density sediment i s formed, consistent with f l o c c u l a t i o n and the t u r b i d i t y f l u c t u a t i o n technique complements t h i s by i n d i c a t i n g a larger average p a r t i c l e size f o r the same polymer dose. Thus at less than complete coverage the HEC may f l o c c u l a t e the cyanazine p a r t i c l e s by a bridging mechanism, whereby a network i s formed by an HEC molecule, simultaneously adsorbing to more than one p a r t i c l e . C o l l o i d a l s t a b i l i t y i s re-achieved at higher polymer doses, as might be expected since at these higher coverages, when the polymer i s loopy, as shown by the EPR studies, some additional s t e r i c s t a b i l i s i n g capacity may be derived from the HEC. For the higher volume f r a c t i o n suspensions, however, i n the range of HEC doses investigated, complete surface coverage i s never achieved, and thus the suspension i s always i n a state of weak f l o c c u l a t i o n as a r e s u l t of the presence of the HEC. Ultrasonic V e l o c i t y Studies. Since there are no techniques presently available f o r monitoring f l o c c u l a t i o n i n concentrated suspensions, we have developed a technique using ultrasound v e l o c i t y scanning i n sedimenting systems which may be applied to f l o c c u l a t i o n monitoring and also following such processes as d i l a t a n t clay formation i n suspension concentrates. In F i g 8 we show the results obtained f o r sedimentation studies on a concentrated (40% volume fraction) suspension of unstructured c o l l o i d a l l y stable cyanazine p a r t i c l e s , where the sample was repeatedly scanned over a period of 105 days. The b u i l d up of d i l a t a n t clay at the c e l l base can quite c l e a r l y be seen, where the p a r t i c l e s pack to the random close packed l i m i t $-0.65), although t h i s value may be influenced by the p o l y d i s p e r s i t y of the particles. In F i g 9 we show a s i m i l a r scan over a comparable time period, but i t may be seen that, at the base of the c e l l , the l i m i t i n g volume f r a c t i o n i n the cyanazine enriched layers i s approximately 0.48. I f the HEC were serving to structure the system simply by the influence i t would have on the v i s c o s i t y of the continuous phase, then we could expect to f i n d a l i m i t i n g volume f r a c t i o n i n the layers at the base of the c e l l l a r g e l y the same as that found i n the unstructured system. Thus the findings f o r the HEC structured systems indicate that the system must be weakly flocculated, and we propose that the f l o c c u l a t i o n i s by a s i m i l a r bridging mechanism to that observed i n the low volume f r a c t i o n suspensions, since the EPR data indicate that the configuration and extent of surface coverage by the adsorbed HEC polymer i n the concentrated suspensions (f-0.4) i s s i m i l a r . Conclusions This study demonstrates quite c l e a r l y that the mode of action of the structure agent HEC i n the system under investigation i s that i t induces controlled f l o c c u l a t i o n by a bridging mechanism. The


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Pesticide Formulations - American Chemical Society

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