Advances in Pesticide Formulation Technology - American Chemical

12 Reduction of Propargite Phytotoxicity Through Spray Drying 1

ROBERT F. PETERSON, JR.

Downloaded by CORNELL UNIV on October 7, 2016 | http://pubs.acs.org Publication Date: June 5, 1984 | doi: 10.1021/bk-1984-0254.ch012

Uniroyal Chemical Company, Bethany, CT 06525

The phytotoxicity of propargite, a sulfite acaricide, can be greatly reduced, while essentially maintaining miticidal activity, by absorbing the propargite on a carrier, adding a polymer latex, and spray drying the resulting mixture. Scanning electron microscope photographs show this process to result in an irregular, incomplete shell which prevents contact of the phytotoxic liquid with the plant, yet allows escape of propargite vapor in sufficient quantity to control mites. Propargite, a sulfite acaricide, was commercialized by Uniroyal i n 1968 under the trade name of Qmite and i s useful i n the control of many species of mites. (1,2) The structure of propargite and some of i t s physical properties are given i n Table I . Propargite i s a specific acaricide, which i s relatively nontoxic to honeybees and many parasitic insects. In addition, predacious mites are not eliminated from treated areas. A l i s t of important mite species controlled by propargite i s given i n Table I I . Use of propargite, however, i s limited by i t s phytotoxicity to certain crops when used at acaricidal concentrations. Table III l i s t s important sensitive and non-sensitive crops.

1

Current address: Rohm & Haas, Inc., Norristown and McKean Roads, Spring House, PA 19477 0097-6156/ 84/ 0254-0163S06.00/ 0 © 1984 American Chemical Society

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

164

ADVANCES IN PESTICIDE FORMULATION TECHNOLOGY

Table I fflffiïïrtfial. and PhYSÎÇfll Properties of Propargite Η

Ο

I

II

C — Ο — S — Ο — CH — C ^ C H 2


c4 Molecular weight Form Odor Vapor pressure Specific gravity Flash point Water, maximum Miscibility Acute Oral LD^-rats Eye i r r i t a t i o n - r a b b i t s Aires IteSt

350 Ligfrt to dark brown viscous liquid Faint solvent odor Less than 1 mm at 20°C 1.085 - 1-115 20°C Pensky-Martens Less than 0.1% Immiscible with water Miscible with organic solvents 2200 mg/kg Severe Negative ,

Table II Mite Species Controlled by PROPARGITE apple rust mite Banks grass mite brown mite citrus red mite citrus rust mite clover mite cyclamen mite European red mite McDaniel spider mite Pacific spider mite peach s i l v e r mite six-spotted mite Southern red mite strawberry spider mite Texas citrus mite two-spotted spider mite

Willamette Pflte

Aculus schlechtendali (Nalepa) Oligonychus pratensis (Banks) Bryobia arborea (Morgan & Anderson) Panonychus c i t r i (McGregor) Phyllocoptruta oleivora (Ashmead) Bryobia praetiosa (Koch) Steneotarsonemus pallidus (Banks) Panonychus ulmi (Koch) Tetranychus medanieli (McGregor) Tetranychus pacificus (McGregor) Aculus cornutus (Banks) Eotetranychus sexmaculatus (Riley) Oligonychus i l i c i s Tetranychus atlanticus (McGregor) Eutetranychus banksi (McGregor) Tetranychus urticae (Koch) Eotetranvchus willamettei (McGregorL


12.

PETERSON

Reduction of Propargite Phytotoxicity

165

Table III


PhytrOtrOxir,it,y of FROPARBTTR Sensitive crops

Non-sensitive crops

Pears Citrus Cotton (smaller than 12-14") Strawberries Hops Cantaloupe

Walnuts Almonds Cotton (larger than 12-14") F i e l d corn Apples Peanuts

Table III i s meant as a guide but does show that phytotoxicity i s dependent not just on plant species but also on the time of application. Young cotton has an unacceptable level of crop damage; the same plants, treated later i n the growing season, are much less sensitive. Citrus i s l i s t e d as a sensitive crop because a very low level of phytotoxicity on the f r u i t i t s e l f results i n a reduction i n quality and hence i n the price paid for the f r u i t . This phenomenon has kept propargite from sales on citrus i n spite of i t s excellent a c t i v i t y against the mites found i n citrus. The acaricidal a c t i v i t y of propargite occurs primarily through vapor action, while the phytotoxic effects seem to arise only from direct contact with the plant leaf or f r u i t i n g body. Conceptually, therefore, a delivery system which could prevent contact with the plant, yet allow the acaricidal vapor to escape, would prevent phytotoxicity and allow use on sensitive crops. Scher (3.) has reviewed the microencapsulation of pesticides and discussed some of the possible ways of achieving such a delivery system. Spray drying, one of these methods, i s very suitable for the production of microcapsules. In a spray dryer, an atomizer i s used to form a spray of small droplets, which then mix with hot gases to evaporate the l i q u i d from the droplets to form a dispersed, dry product. This dry product must then be separated from the exhaust gases. Figure I shows a schematic diagram of a spray dryer.



Slurry


Atomizer

» ^

- — Warm • Air Exhaust Air

Spray dried

t Powder

Figure 1. Spray drying system


12.

PETERSON

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Materials and Methods The following materials were obtained from the sources l i s t e d and used without further purification.


Material

Source

Trade Name

Propargite Qmite Diatomaceous earth Microcel Ε Sodium lignosulfonate Marasperse CBOS-3 Octyl phenol polyoxyethanol, adsorbed on magnesium carbonate Triton AG 120 Polymerized sodium salt of a l k y l naphthalenesulfonic acid Daxad 11 Dioctyl ester of sodium Aerosol 0TB sulfosuccinic acid Polyvinyl acetate (latex) Polyco 117-SS

Uniroyal Chemical Johns-Manville American Can Co.

Rohm & Haas

W. R. Grace American Cyanamid Borden Co.

A 3-1/2 foot diameter "Laboratory Spray Dryer" made by Bowen Engineering, Somerville, NJ, equipped with a centrifugal atomization nozzle, was used for these experiments. Table IV gives our procedure for the preparation of spray dried propargite. Table IV Preparation of Encapsulated Propargite Step

Ingredients

Equipment

1

Propargite Diatomaceous earth Wetting and dispersing surfactants

Ribbon blender

Product of step 1

Slurry tank (with agitator)

Polyvinyl acetate latex Water Wetting and dispersing surfactants Product of step 2

Spray dryer

Results and Discussion The phytotoxicity and acaricidal activity of this new formula tion i s shown i n Table V. A commercial wettable powder formulation of propargite, Qmite 30W, was used for comparison purposes.


168


_

Table V

'


Phytotoxicity and Acaricidal Activity Rate of propargite, parts per m i l l i o n (ppm)

8000

2000

500

100

20

Phytotoxicity (a) of spray dried formulation

15

2

0

—

—

Phytotoxicity (a) of wettable powder

95

20

1

—

—

% control of mites (b) spray dried formulation

—

—

100

77

69

100

89

50

% control of mites (b) wettable powder

(a) Phytotoxicity: % of damaged tissue on primary leaves of cowpeas (Vigna sinensis) 7 days after treatment (b) % control of mites: Mites loaded 1 day after spraying, scored (using Abbott's formula) one week later. Test species: Tetranychus urticiae (Koch). As the table shows, phytotoxicity of the spray dried formulation i s much lower than the conventional formulation, while acaricidal activity i s not significantly reduced. Figure 2 shows scanning electron microscope pictures, at 300X magnification, of the two formulations. The spray dry formulation shows an irregular, incomplete shell around the p a r t i c l e . The atomization system of the spray dryer produces droplets containing polyvinyl acetate micelles i n water, surrounding the particles of propargite absorbed on carrier. As the droplet dries i n the hot, turbulent gases of the spray dryer, the water i s lost (4) and the polyvinyl acetate dries to an irregular coating around the c a r r i e r , as seen i n the photomicrograph. The conditions of spray drying are optimized so as to provide the least possible degree of agglomeration, so that the particle size of the resulting powder i s kept as small as possible. We believe that the shell acts as a physical barrier preventing contact of propargite with the leaf; thus, the presence of a shell reduces phytotoxicity. Because the shell i s porous and incomplete, the polyvinylacetate shell does not prevent the



Figure 2.

Propargite formulations (300X): r i g h t , spray dried; l e f t , wettable powder


On

—

I

«3

S. δ*

ST

2

ι

Ο

m

m Η

-α


Figure 3.

E f f e c t o f s h e l l weight on p r o p a r g i t e e f f i c a c y and p h y t o t o x i c i t y


ο r Ο o

H m η

δ

c r

Ο ?o

*n

m

5

tn H η

3

m

Ο

α

>

12.

PETERSON

171


escape of propargite vapor. Mite control, then, i s no different than for conventional formulations of propargite. The spray dry formulation i s therefore not a "controlled release" formulation as we have seen no increase i n the length of control provided by a given amount of propargite. The effect of increasing shell weight on efficacy and phytotoxicity i s shown i n Table V I . The test plant was cowpeas (Vigna sinensis). Table VI


Effect of Shell Weight Weight of s h e l l , %

0 (conventional WP) 10 15 20 30 45

% control 100 ppm 55 87 80 58 70 27

% control 20 ppm 45 45 25 7 18 5

% phyto % phyto 8000 ppm 2000 ppm 95 60 15 0 5 0

45 15 3 1 0 0

These data points are shown graphically i n Figure 3. The reduction i n % control with increasing s h e l l weight i s almost linear. In contrast, there i s a sharp drop i n phytotoxicity between 10% and 20% by weight polyvinyl acetate. Evidently, 10% by weight of polyvinyl acetate i s not enough to surround the particle i n sufficient amount to prevent contact of propargite with the leaf surface, while a 20$ shell i s thick enough to prevent contact, hence lowering phytotoxicity.

Literature Cited 1. R.A. Covey, A.L. Smith and W. L. Hubbard, U.S. patent 3,311,534 2. R.A. Covey, A.L. Smith and W.L. Hubbard, U.S. patent 3,463,859 3. H.B. Scher, American Chemical Society Symposium Series #53 (1977) page 126 ff. 4. K. Masters, Spray Drying Handbook, 3rd edition, John Wiley and Sons (1979), page 291 ff. RECEIVED March 16, 1984


Advances in Pesticide Formulation Technology - American Chemical

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