Advances in Pesticide Formulation Technology - American Chemical

17 Physical Properties Determining Chargeability of Pesticide Sprays

Downloaded by UNIV OF CALIFORNIA SAN DIEGO on April 14, 2016 | http://pubs.acs.org Publication Date: June 5, 1984 | doi: 10.1021/bk-1984-0254.ch017

S. EDWARD LAW Driftmier Engineering Center, University of Georgia, Athens, GA 30602

From a formulation viewpoint, the several technical approaches to the droplet-charging phase of the electrostatic pesticide-spraying process are shown to depend primarily upon liquid electrical resis tivity and dielectric constant. Ionized-field charging of low permittivity oil-based sprays theoretically attains at least half the charge level of that imparted to aqueous-based pesticide spray having ca. a 40-fold greater dielectric constant. Theoretically, electrostatic induction is shown to satisfactorily charge spray liquids exhibiting resistivity values over a 10 -fold range from ca. 10 ohm m downward. For direct electrostatic atomization and charging of hydrocarbon liquids by 2-electrode systems, precise formulation to assure that resistivity values lie within a comparatively narrow 10 - 10 ohm m region is required. Experi mentally measured resistivity values for common commercial pesticide formulations were typically found to range from ca. 10 to 1 ohm m for water: pesticide dilutions of 0:1 to 80:1, respectively. 6

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Incorporation of e l e c t r i c f o r c e f i e l d s i n t o p e s t i c i d e spray a p p l i c a t i o n has been shown to g r e a t l y increase the mass-transfer e f f i c i e n c y of the b a s i c d r o p l e t d e p o s i t i o n process (1-3). The most elementary statement of the c o n d i t i o n s necessary f o r such e l e c t r i c - f o r c e augmentation i s F Ρ

= q Ε Ρ

(1)

where F i s the e l e c t r i c a l f o r c e i n newtons a c t i n g upon a spray droplet^having net f r e e charge of q coulombs, and Ε i s the e l e c t r i c f i e l d i n v o l t s per meter ^experienced at the d r o p l e t l o c a t i o n . Previous studies have considered the t e c h n i c a l

0097-6156/ 84/ 0254-0219506.00/ 0 © 1984 American Chemical Society

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


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ADVANCES IN PESTICIDE FORMULATION TECHNOLOGY

requirements and r e l a t i v e magnitudes of the e l e c t r i c f i e l d s E* a s s o c i a t e d with image charges induced i n t o t a r g e t boundries, with e x t e r n a l l y a p p l i e d electrodes> and with the space charge d e l i v e r e d by the airborne spray cloud i t s e l f (4). A d d i t i o n a l l a b o r a t o r y s t u d i e s u s i n g tracer-tagged sprays have documented that d r o p l e t charge l e v e l s corresponding to 1 mC/kg or greater a c t i n g i n con j u n c t i o n with such e l e c t r i c f i e l d s g e n e r a l l y provide very s i g n i f i c a n t increases i n spray d e p o s i t i o n e f f i c i e n c y (5,6). The o b j e c t i v e o f t h i s paper i s to b r i e f l y review d r o p l e t - c h a r g i n g methods and, hence, to d e f i n e f o r the p e s t i c i d e formulator those p h y s i c a l p r o p e r t i e s which determine the degree of e l e c t r i c a l c h a r g e a b i l i t y of l i q u i d s f o r use i n the e l e c t r o s t a t i c spraying process. Theoretical Limitations In the t e c h n i c a l approaches described below f o r imparting d r o p l e t charge qp, t h e i r l e v e l s of performance can be a s c e r t a i n e d by comparison with t h e o r e t i c a l l i m i t s on the maximum charge which can e x i s t on an i s o l a t e d p a r t i c u l a t e . Two such l i m i t s a r e e s t a b l i s h e d by: (a) d i e l e c t r i c breakdown of the a i r at the h i g h l y curved surface of the charged s o l i d or l i q u i d p a r t i c u l a t e with subsequent gaseous discharge, -i.e.., the i o n emission l i m i t ; and (b) hydrodynamic i n s t a b i l i t y due to e l e c t r i c a l f o r c e s a c t i v e w i t h i n the periphery of a charged l i q u i d d r o p l e t exceeding surface tension f o r c e s , with subsequent e j e c t i o n of a p p r e c i a b l e charge and mass, -i.e., the Rayleigh l i m i t ( 7 ) . As seen i n Figure 1, the Rayleigh charge l i m i t as c a l c u l a t e d by the equation

= 8π

fz

ο

Fr

3/2

(2)

Ρ

i s dominant f o r d r o p l e t s of l i q u i d having surface t e n s i o n values Γ t y p i c a l of aqueous and o i l - b a s e d p e s t i c i d e sprays. As surface t e n s i o n ranges to a low value of, say, 20 mN/m f o r s u r f a c t a n t laden l i q u i d s , the s t a b i l i t y l i m i t on d r o p l e t charge reduces to approximately 52% of i t s common value f o r water (Γ * 72 mN/m). Thus, f o r tank p r e p a r a t i o n s o f any given p e s t i c i d e formulation, Equation 2 t h e o r e t i c a l l y e s t a b l i s h e s the maximum a t t a i n a b l e l e v e l of c h a r g e a b i l i t y as a f u n c t i o n of l i q u i d surface t e n s i o n . Droplet Charging

Methods

F i e l d - p r o v e n methods f o r imparting charge qp to p e s t i c i d e sprays are: (a) i o n i z e d - f i e l d d r o p l e t charging of both conductive and non-conductive l i q u i d s ; (b) e l e c t r o s t a t i c - i n d u c t i o n d r o p l e t charging of conductive l i q u i d s ; and (c) d i r e c t e l e c t r o s t a t i c atomization and charging of non-conductive l i q u i d s . When con s i d e r e d i n r e l a t i o n to t e c h n i c a l requirements o f the v a s t l y d i f f e r i n g p e s t i c i d e - a p p l i c a t i o n s i t u a t i o n s encountered i n



17.

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Chargeability Determination of Sprays

221

a g r i c u l t u r e , each droplet-charging method possesses i t s unique advantages as w e l l as disadvantages; they are complementary approaches. Figure 2 i l l u s t r a t e s i n concept these droplet-charging methods. Shown i n a x i a l l y symmetrical geometry i s a continuous j e t J of l i q u i d i s s u i n g at a v e l o c i t y ν from a f l u i d nozzle Ν and d i r e c t e d toward an o u t l e t end near which a sharply pointed d i s charge e l e c t r o d e Ρ i s l o c a t e d . By i n t e r a c t i o n of e x t e r n a l l y a p p l i e d energy the continuous l i q u i d j e t J may be d i s r u p t e d i n t o d i s c r e t e a i r b o r n e d r o p l e t s w i t h i n a d r o p l e t - p r o d u c t i o n zone Ζ intermediate i n l o c a t i o n to the nozzle Ν and the p o i n t P. Coaxial with t h i s j e t and zone i s a c y l i n d r i c a l e l e c t r o d e C which can i n f l u e n c e the e l e c t r i c - f i e l d d i r e c t i o n and i n t e n s i t y i n the neighborhoods of zone Ζ and p o i n t P. By the connection of the conductors L., L~ and L^ to v a r i o u s combinations of e l e c t r i c a l p o t e n t i a l , tne three charging methods can be e f f e c t e d as described below. Ionized F i e l d Droplet Charging. When conductors L^ and L 2 are grounded and a s u f f i c i e n t l y high d.c. p o t e n t i a l i s a p p l i e d to conductor L 3 , d i e l e c t r i c breakdown of the a i r immediately sur rounding the metal point Ρ w i l l r e s u l t . For the c y l i n d r i c a l geometry shown, a s e l f - s u s t a i n i n g corona discharge current w i l l , thus, flow between Ρ and C such that the major p o r t i o n of the c y l i n d r i c a l gap i s occupied by u n i p o l a r a i r ions t r a v e l i n g outward along the r a d i a l e l e c t r i c - f i e l d l i n e s to the n o n - i o n i z i n g e l e c trode C. E i t h e r s o l i d or l i q u i d p a r t i c u l a t e s of diameters l a r g e r than approximately 0.5 ym t r a v e l i n g through t h i s i o n i z e d - f i e l d r e g i o n can t h e o r e t i c a l l y acquire by i o n attachment a s a t u r a t i o n charge dependent upon the p a r t i c u l a t e ' s d i e l e c t r i c constant K, i t s surface area, and the e l e c t r i c a l c h a r a c t e r i s t i c s of the corona discharge. ( D i f f u s i o n - c h a r g i n g theory a p p r o p r i a t e l y a p p l i e s f o r p a r t i c u l a t e s smaller than 0.5 ym here, but t h i s s i z e realm i s of l i t t l e consequence f o r p e s t i c i d e spray-charging development.) The f r a c t i o n f of the s a t u r a t i o n charge a c t u a l l y a t t a i n e d by the p a r t i c u l a t e depends upon the residence time t , and the concentra t i o n Ν and m o b i l i t y of the ions i n the charging f i e l d . I o n i z e d - f i e l d p a r t i c u l a t e - c h a r g i n g theory has been developed mathematically (8) and i s summarized by the f o l l o w i n g equations 2

q = f Γΐ + 2 ^ - Ί 4πε Ε r ^p I K+2 J ο ο ρ

(3)

(Nek./4ε ) t (Nek./4ε ) t + 1 1 ο r v

K

'

As a p r a c t i c a l matter, f r a c t i o n a l charge values of f = 0.5 s a t u r a t i o n may be imparted i n one p a r t i c l e - c h a r g i n g time constant of t = kzJlSéb.. t y p i c a l l y of s e v e r a l m i l l i s e c o n d s d u r a t i o n . For r




DROPLET RADIUS r

p

,(pm)

Figure 1. T h e o r e t i c a l charge l i m i t s f o r airborne p e s t i c i d e d r o p l e t s as f u n c t i o n s of d r o p l e t r a d i u s and l i q u i d surface tension (25°C).

Figure 2. Conceptual r e p r e s e n t a t i o n o f spray-droplet charging methods. (Reproduced with permission from Ref. 10. Copyright 1978, American S o c i e t y o f A g r i c u l t u r a l Engineers.)



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Chargeability Determination of Sprays

223

t y p i c a l corona charging c o n d i t i o n s , the magnitude of d r o p l e t s a t u r a t i o n charge i s p l o t t e d i n Figure 1 as a f u n c t i o n of d r o p l e t r a d i u s r« f o r aqueous spray (K = 80). As f a r as the p h y s i c a l p r o p e r t i e s of a p e s t i c i d e m a t e r i a l are concerned i n t h i s charging process, the l i q u i d ' s d i e l e c t r i c con stant Κ i s the main f a c t o r which determines d r o p l e t charge t r a n s f e r . The bracketed Κ expression i n Equation 3 i s a measure of the degree to which the i o n i c charging e l e c t r i c - f i e l d l i n e s are concentrated onto the airborne p a r t i c u l a t e s . As seen i n Figure 3, the expression ranges i n value from approximately 1.5 f o r good d i e l e c t r i c s such as o i l s to a maximum value of 3 f o r conducting p a r t i c l e s . Thus, t h e o r e t i c a l l y even d r o p l e t s of a l i q u i d d i e l e c t r i c m a t e r i a l (such as many u n d i l u t e d p e s t i c i d e formulations) could be charged at l e a s t h a l f as w e l l as h i g h l y conductive m e t a l l i c p a r t i c l e s or water d r o p l e t s by t h i s i o n i z e d - f i e l d charging process. Induction Droplet Charging. I f the gaseous-discharge e l e c t r o d e i s removed and i f a p o s i t i v e p o t e n t i a l i s a p p l i e d to the c y l i n d r i c a l e l e c t r o d e C i n F i g u r e 2 by connection of a v o l t a g e source between conductors and then t h e o r e t i c a l l y f o r any l i q u i d having non-zero e l e c t r i c a l c o n d u c t i v i t y an excess negative charge w i l l accumulate on the grounded l i q u i d j e t J . T h i s charge t r a n s f e r r e s u l t s from the e l e c t r o s t a t i c i n d u c t i o n of e l e c t r o n s onto the a x i a l j e t i n order to maintain i t at ground p o t e n t i a l i n the presence of the nearby charged c y l i n d r i c a l e l e c t r o d e . I n d i v i d u a l d r o p l e t s formed from the surface of t h i s n e g a t i v e l y charged continuous j e t w i l l depart with a net negative charge provided that the d r o p l e t - f o r m a t i o n zone Ζ i s subject to the inducing e l e c t r i c f i e l d a c t i n g between the c y l i n d e r and the j e t . Gauss' law i n d i c a t e s that maximum d r o p l e t charging should occur f o r the d r o p l e t - p r o d u c t i o n zone l o c a t e d at the r e g i o n which provides maximum f i e l d strength at the t e r m i n a l surface o f the j e t . In theory, the l e v e l of d r o p l e t charge imparted by the e l e c t r o s t a t i c - i n d u c t i o n process depends h e a v i l y upon the r e l a t i v e time r a t e of charge t r a n s f e r to the d r o p l e t - f o r m a t i o n zone as compared with the time r e q u i r e d f o r d r o p l e t formation. The charge-transfer c a p a b i l i t y by conduction from the grounded metal f l u i d nozzle Ν through the l i q u i d j e t J depends upon the e l e c t r i c a l p r o p e r t i e s of the l i q u i d forming the continuous j e t . For the p e s t i c i d e formulator, t h i s s p r a y - l i q u i d c h a r a c t e r i s t i c may be s p e c i f i e d by the charge-transfer time constant (9) or charge r e l a x a t i o n time τ which i s a f u n c t i o n of the e l e c t r i c a l conduc t i v i t y σ and the p e r m i t t i v i t y ε of the l i q u i d as τ = ε/σ

In terms of the spray l i q u i d ' s d i e l e c t r i c constant Κ and r e s i s t i v i t y p, t h i s time constant becomes


(5)

224


τ = Κ ρε

(6)

ο

T h e o r e t i c a l l y spray l i q u i d s having charge-transfer time constants τ l e s s than the length of time t f which c h a r a c t e r i z e s droplet formation should be compatible with the e l e c t r o s t a t i c i n d u c t i o n charging process, while l i q u i d s having T>tf could not be s a t i s f a c t o r i l y charged by t h i s method. For the g e n e r a l i z e d charger arrangement of Figure 2, the c h a r a c t e r i s t i c d r o p l e t formation time i s estimated to be approximately 1.6 msec (10). This a v a i l a b l e charging time i s more than 22 χ 10 longer than the 1=70 nsec value c a l c u l a t e d by Equation 6 f o r aqueous sprays (e.g., K-80, p=10 ohm m). A p e r i o d of f i v e time constants i s g e n e r a l l y considered adequate to e f f e c t greater than 99% of the charge t r a n s f e r a t t a i n a b l e by e l e c t r o s t a t i c i n d u c t i o n f o r a given spray l i q u i d . Thus, f o r water-based sprays Figure 4 i n d i c a t e s that charge-transfer l i m i t a t i o n (as defined by tf4.5 χ 10 ohm m. The c o r r e s ponding value f o r o i l - b a s e d sprays (e.g., K=5) would be ρ>7.2 χ 10 ohm m. Spray l i q u i d s l e s s r e s i s t i v e than these values should charge s a t i s f a c t o r i l y by the e l e c t r o s t a t i c - i n d u c t i o n method. I t should be mentioned that i t i s p o s s i b l e to a l s o e f f e c t the e l e c t r o s t a t i c - i n d u c t i o n charging process i n a reverse manner to that described above; that i s , a negative p o t e n t i a l may be d i r e c t l y a p p l i e d to L j while L 2 i s grounded to permit the c y l i n d e r C to serve as a f i e l d i n t e n s i f y i n g e l e c t r o d e (11) . T h i s v a r i a t i o n of the e l e c t r o s t a t i c - i n d u c t i o n method n e c e s s i t a t e s a high degree of i s o l a t i o n of the e l e c t r i f i e d n o z z l e and i t s l i q u i d . However, a l l other formulation c r i t e r i a are as p r e v i o u s l y s t a t e d .


3

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E l e c t r o s t a t i c Atomization. I f the f o l l o w i n g m o d i f i c a t i o n s are made i n the apparatus of F i g u r e 2, then e l e c t r i c a l f o r c e s can be u t i l i z e d to s t r e s s a j e t of l o w - c o n d u c t i v i t y l i q u i d s u f f i c i e n t l y to atomize (and charge) the l i q u i d with no a d d i t i o n a l energy input (12): (a) remove p o i n t P; (b) reduce the length t of the earthed f i e l d - i n t e n s i f y i n g e l e c t r o d e C and p o s i t i o n i t upstream from the d r o p l e t - p r o d u c t i o n zone Z; (c) l e t the j e t approach c a p i l l a r y dimensions by reducing j e t - o r i f i c e r a d i u s r j or by a l t e r i n g i t to an annulus; and (d) apply a high d.c. p o t e n t i a l to the l i q u i d j e t through L^. T h i s device then e s s e n t i a l l y becomes a 2-electrode e l e c t r o s t a t i c atomizer appropriate f o r use with o i l formulations. S u c c e s s f u l e x p l o i t a t i o n of t h i s d r o p l e t atomization and charging method r e q u i r e s very r i g o r o u s s p e c i f i c a t i o n s regarding the l i q u i d ' s e l e c t r i c a l and v i s c o e l a s t i c p r o p e r t i e s . For the type spray-charging device described, p e s t i c i d e - l i q u i d r e s i s t i v i t y i s g e n e r a l l y l i m i t e d to the f a i r l y narrow 10 - 10 ohm m r e g i o n of v a l u e s . A somewhat more recent approach to e l e c t r o s t a t i c atomization and charging has been developed f o r hydrocarbon f u e l s i n v a r i o u s combustion systems (13) . T h i s method employs d i r e c t charge i n j e c t i o n i n t o the continuous stream of hydrocarbon l i q u i d by means of 6


8


Figure 3. Effect of spray-liquid d i e l e c t r i c constant upon chargingf i e l d concentration factor Jj+2(K-1)/(K+2)Q associated with ionized-field droplet charging.

3

4

5

6

7

8

Figure 4. Charge-transfer time constants characterizing spray liquids as functions of l i q u i d e l e c t r i c a l r e s i s t i v i t y and d i e l e c t r i c constant. (Reproduced with per mission from Ref. 10. Copyright 1978, American Society of Agricultural Engineers.)

2

ΙΟ"" 1 I I I 1 1 1 1 K> Ι Ο Ι Ο Ι Ο I0 I0 I0 I0 9 » SPRAY LIQUID RESISTIVITY, (ohm cm)


9

^ &

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a submerged f i e I d - e m i s s i o n e l e c t r o n gun i n c o r p o r a t i n g a setaceous e l e c t r o n - e m i t t e r surface of % ym tungsten f i b e r s developed i n a UO2 matrix at an area d e n s i t y of approximately 4 χ 10 /cm . With t h i s "Spray Triode ' c l a s s of e l e c t r o s t a t i c - a t o m i z i n g device, K e l l y (14) r e p o r t s that s p r a y - l i q u i d e l e c t r i c a l c o n d u c t i v i t y does not fundamentally determine the c h a r a c t e r i s t i c s of the charged spray subsequently generated, and that t h e o r e t i c a l l y no minimum l i q u i d c o n d u c t i v i t y l i m i t i s a s s o c i a t e d with the o p e r a t i o n of such e l e c t r o s t a t i c atomizers based upon the d i r e c t c h a r g e - i n j e c t i o n method: " U n i v e r s a l spray behavior occurs f o r a l l f l u i d s having v i s c o s i t i e s η 0.1/Γ. In t h i s spray regime d r o p l e t s i z e d i s t r i b u t i o n i s narrow (

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