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«: AGROCHEMICALS: FATE IN FOOD AND THE ENVIRONMENT PROCEEDINGS OF A SYMPOSIUM, ROME, 7 - 1 1 JUNE 1982 JOINTLY ORGANIZED BY IAEA AND FAO l^J I N T E R ...»

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[5] PILLMOR, J.B., GAUNT, J.K., "The behaviour and mode of action of phenoxyacetic acid in plants", Progress in Pesticide Biochemistry I (HUTSON, D.H., ROBERTS, T.R., Eds), J. Wiley and Sons, Chichester (1981) 147.

IAEA-SM-263/38 221 ROBERTS, T.R., "The metabolism of synthetic pyrethroids in plants and soils", Progress in Pesticides Biochemistry I (HUTSON, D.H., ROBERTS, T.R., Eds), J. Wiley and Sons, Chichester (1981) 115.

METCALF, R.L., MARCH R.B., FUKUTO, J.R., MAXON, M.G., Behaviour of systoxisomersin bean and citrus plants, J.Econ. Entomol. 47 (1954) 1045.

METCALF, R. L„ FUKUTO, T.R., MAXON, M.G., The nature and significance of systox residues in plant materials, J. Econ. Entomol. 48 (1955) 364.

FUKUTO, T.R., WOLFE, J.P., METCALF, R.L., MARSH, R.B., Plant metabolism of the thiol ester of systox (demeton), J. Econ. Entomol. 49 (1956) 147.

FUKUTO, T.R., WOLFE, J.P., METCALF, R.L., MARSH, R.B., Identification of sulfone plant metabolite of the thionoisomer of systox, J. Econ. Entomol. 50 (1957) 399.

METCALF, R.L., FUKUTO, T.R., MAXON, M.G., MARSH, R.B., Plant metabolism of deithiosystos and Thimet, J. Econ. Entomol. 50 (1957) 338.

AKRAM, M., AHMAD, S., FORGASH, A.J., Metabolism of phosphoric acid,

0.0.dimethy-0-(6-ethoxy-2-ethyl-4-pyrimidinyl) ester (Etrimfos), in bean and corn plants, J. Agrie. Food Chem. 26 4 (1978) 925.

LORD, K.A., WHEELER, A.W., Uptake and movement of 14C-chlormequat chloride applied to leaves of barley and wheat, J. Exp. Bot. 32 (1981) 599.

BROMILOW, R.H., BAKER, R.J., FREEMAN, M.A.H., GOROG, K„ The degradation of aldicarb and oxamyl in soil, Pestic. Sci. II ( 1980) 371.

BARTLEY, W.J., ANDRAWES, N.R., CHANCEY, E.L., BAGLEY, W.P., SPURR, H.W., Metabolism of Temik aldicarb pesticide [2-methyl-2-(methylthio)propionaldehyde 0-(methylcarbamoyl)oxime] in the cotton plant, J. Agrie. Food Chem. 18 3 (1970) 446.

ANDRAWES, N.R., BAGLEY, W.P., HERRETT, R.A., Metabolism of 2-methyl-2methylthio) propionaldehyde O-(methylcarbamoyl) oxime (Temik aldicarb pesticide) in potato plants, J. Agrie. Food Chem. 19 4 (1971) 731.

RONCHARD, J., MOONS, C., MEYER, J.A., The metabolism of [14C]-Aldicarb in the leaves of sugar beet plants, Pestic. Sci. 11 (1980) 483.

RONCHARD, J., MOONS, С., MEYER, J.A., The metabolism of [14C]-Aldicarb in the root of sugar beet plants, Pestic. Sci. 12 5 (1981) 548.

KU, C.C., KAPOOR, I.P., ROSEN, J.D., Metabolism of cytrolane (mephosfolan) systemic insecticide [(diethoxyphosphenyl)dithioimidocarbonic acid, cyclic propylene ester] in a simulated rice paddy, J. Agrie. Food Chem. 26 6 (1978) 1352.

[20] BELASEO, I.J., HAN, J.C-Y., CHIZANOWSKI, R.L., BAUDE, F.J., Metabolism of [ 14 C]-Cymoxanilin grapes, potatoes and tomatoes, Pestic. Sci. 12 (1981) 355.

BEGUM, S., SCHEANERT, I., HAQUE, A., KLEIN, W., KORTE; F., Conversion of [14C]-pentachloronitrobenzene in onions, Pestic. Biochem. Physiol. II (1979) 189.

[22] LAMOUREUX, G.L., RUSNESS, D.G., Pentachloronitrobenzene metabolism in peanut.

1. Mass spectral characterization of seven glutathion-related conjugates produced in vivo or in vitro, J. Agrie. Food Chem. 28 (1980) 1057.

LAMOUREUX, G.L., RUSNESS, D.G., Pentachloronitrobenzene metabolism in peanut.

2. Characterization of chloroform metabolites formed in vivo, J. Agrie. Food Chem.

2 8 ( 1 9 8 0 ) 1070.

[24] LAMOUREUX, G.L., RUSNESS, D.G., In vitro metabolism of pentachloronitrobenzene to pentachloromethylthiobenzene by onion: characterization of glutathione S-transferase, cystein C-S lyase and S-adenosylmethionine methyl transferase activities, Pestic. Biochem.

Physiol. 141(1980) 50.

222 LORD [25] FEUNG, C.S., LOARCH, S.L., HAMILTON, R.H., MUMMA, R.O., Comparative metabolic fate of 2,4,dichlorphenoxyacetic acid in plants and plant tissue culture, J. Agrie. Food Chem. 26 5 (1978) 1064.

[26] WRIGHT, A.N., ROBERTS, T.R., DUTTON, A.J., DOIG, M.V., The metabolism of cypermethrin in plants: the conjugation of the cyclopropyl moiety, Pestic. Biochem.

Physiol. 13 1 (1980) 71.

[27] ROBERTS, T.R., WRIGHT, A.N., The metabolism of 3-phenoxy-benzyl alcohol, a pyrethroid metabolite, in plants, Pestic. Sci. 1 2 2 ( 1 9 8 1 ) 1 6 1.

[28] BULL, D.L., IVIE, G.W., Fate of diflubenzuron in cotton, soil and rotational crops, J. Agrie. Food Chem. 26 3 (1978) 515.

[29] MANSAGER, E.R., STILL, G.G., FREAR, D.S., Fate of [14C]-diflubenzuron on cotton and in soil, Pestic. Biochem. Physiol. 12 2 (1979) 172.

[30] KNOWLES, C.O., FRANKLIN, E.J., Metabolism of diflubenzuron by spider mites and bean plants, Pestic. Sci. 12 2 (1981) 133.

[31] KIRKPATRICK, D., BIGGS, S.R., CONWAY, В., FINN, C.M., HAWKINS, D.R., HONDA, T., ISHIDA, M., POWELL, P.P., Metabolism of N-(2,3-dichlorophenyl) 3,4,5,6-tetrachlorophthamic acid (Techlofthalan) in paddy soil and rice, J. Agrie.

Food Chem. 29 (1981) 1149.

[32] NEWSOME, W.H., SHIELDS, J.B., A radioimmunoassay for benomyl, methyl-2benzimidazole in food crops, J. Agrie. Food Chem. 29 (1981) 220.

[33] ERCEGOVICH, C.D., VALLEJO, R.P., GETTIG, R.R., WOODS, L., BOGUS, E.R., MUMMA, R.O., Development of radioimmunoassay for parathion, J. Agrie. Food Chem. 2 9 ( 1 9 8 1 ) 559.

IAEA-SM-263/31

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CONTROLLED RELEASE FORMULATIONS OF AGRICULTURAL CHEMICALS.





Chemical agents of agricultural importance can be formulated in a polymeric matrix and their slow continuous release effected through internal transport mechanisms or triggered by external environmental factors. The five basic release systems are all amenable to agricultural usage. The major advantages include reduction of the total quantity of agent necessary, economy in labour through increasing the between-application intervals, greater safety in handling and dissemination of hazardous materials, and reduced environmental impact arising from usage of non-persistent pesticides. It is noted that continuous target exposure to ultra-low pesticide concentration provides much lower insect or weed control at agent level than conventional technology. An increasing number of commercial controlled release products indicates not only a growth in technology but also acceptance on the market. Controlled release systems lend themselves to the scientific study of agricultural agent movement through the soil and plant tissue in that the dispensing unit serves as a relatively stationary retrievable focal source for agent emission. Measurement of agent emission into the ambient environment is thus facilitated through analysis of dispenser residue.

–  –  –

Agent emission ceases when the carrier content drops to a level equivalent to the available free volume. This release mechanism has been observed with other agents, organophosphates and carbamates in particular, and in various polymers [ 5 - 9 ].

More recently three-phase systems have been reported [ 1 0 ] involving agent A in solution in agent B, with both solutes in a carrier.

A variation o f the carrier concept has been used in the membrane/reservoirtype systems in wide commercial application as anti-bacterial fabrics, contact insecticidal tapes, etc. [ 1 1 - 1 3 ].

In 1964 one o f the authors formulated specific organotin anti-fouling agents in elastomers and developed the diffusion-dissolution mechanism o f release [14].

The resulting product has exhibited over 10 years biologically effective life in commercial usage [15]. The technique was extended to include molluscicides [16], membrane/reservoir systems for even longer agent emission [ 1 7 ] and three-phase systems based on a monolithic absorbant reservoir mechanically dispersed within a rubber matrix [ 18]. The diffusion-dissolution release mode was found applicable to aquatic insect larvicides [19], aquatic herbicides [20, 21 ], schistosome larvicides [22] and bactericides [23].

The diffusion-dissolution mechanism required agent solubility in the selected elastomer. Non-soluble materials will not release from polymers by this method and some sort o f leaching mechanism had to be developed, as with most antifouling paints.

It was found possible to incorporate copper sulphate at a 30% or higher loading in an ethylene-propylene-diene terpolymer and achieve 6 to 8 months continuous Си++ emission, provided a secondary co-leachant was used to adjust interfacial pH [24]. It was discovered that two materials could be simultaneously leached from an elastomer.

One basic problem retarding commercial acceptance o f controlled release agent/elastomer systems was not efficacy, which has been well demonstrated under field conditions, but the high cost o f processing. Use o f thermoplastics would be ameliorative and studies began in 1975 with this view in mind.

1.2. Controlled release mechanisms Other than the 'carrier type' mechanism there are four distinct methods through which a chemical agent can be slowly released from a polymeric matrix.

Each method will be summarized in the following sub-sections.

1.2.1. Micro-encapsulation

–  –  –

liquid enveloping film encompassing a solid, liquid or gaseous core. This technology arose in 1939 and underlines hundreds o f commercial products. There are over 180 US patents covering formulations, processes and uses. In the late 1960s it was applied to pesticides. Efficacy depends on the agent molecule leaving the core area and moving to the target. This can occur through vapour diffusion o f the agent permeating the envelope or destruction o f the enveloping and protecting film by chemical or physical natural processes. Micro-encapsulation o f pesticides provides continuous agent emission for several weeks, efficacy and improved safety to humans involved in dispersal operations.

1.2.2. Pendent substitution

This technique consists o f chemically bonding a pest control agent to a polymeric backbone. The bond in question is cleaved through natural processes such as hydrolysis and the agent molecule emitted into the surrounding media, usually water. The technology is new and, except for anti-fouling agents, has yet to be evaluated under rigorous field conditions.

1.2.3. Monolithic agent incorporation in polymers

In this technology a pesticidal agent is incorporated in a polymeric material under unique conditions. Polymer selection is limited to elastomers wherein the agent is soluble (elastomers are highly viscous liquids obeying the laws pf the liquid state). Useable agents are liquids or solids soluble to an appropriate degree in the elastomeric matrix. On addition o f the agent to the elastomer a condition of true solubility exists. Agent release occurs as follows: solute molecules on or near the elastomer/medium interface pass into the medium through volatilization or dissolution processes. This occurrence creates a localized solution disequilibrium and molecules internal to the elastomer migrate under solution pressure to the depleted elastomer surface where the loss process is continuous. Molecules move as a liquid or solid by diffusion processes.

The mechanism is termed diffusion-dissolution in that essential agent migration occurs through diffusion and loss into the ambient media occurs through the passing o f the agent from solution in the elastomer to the external environment.

Elastomers may be cross-linked or not, although in the practical sense the necessary physical properties necessitate cross-linking (vulcanization). Through proper use o f compounding additives, such as carbon black, and proper vulcanization as regards heat history (i.e. degree o f cross-linking) both the solubility and diffusion rate o f the agent in the elastomer matrix is controllable.

This technology arose with the need for long-term anti-fouling coverings but was quickly extended into the public health area through development o f molluscicidal [16, 22, 2 5 ] and insecticidal [19] compounds.

226 CARDARELLI and CARDARELLI

1.2.4. Monolithic agent incorporation in plastics

Plastic and elastomers vary greatly in thermodynamic and kinetic properties, although both categories consist o f high molecular weight polymers. Thermosetting plastics are usually solids at r o o m temperature, and the cross-linking process, once it has occurred, is not reversible. Thermoplastic materials are usually solids but can be liquids at ambient temperatures. Cross-linking is reversible.

Also, materials called 'plastomers' exist which combine some elastomeric properties with some plastic properties in the same molecule. Pesticidal agent solubility in plastics is very low or non-existent. Consequently, the diffusion-dissolution release mechanism is not attainable. Release must arise from some other modus operandi.

It was discovered that a leaching-type mechanism would allow the continuous release o f an agent into the water or soil provided a proper porosity was induced in the dispenser. Thus, a formulation additive was necessary that would enhance porosity. It was known from the earlier art that a secondary co-leachant could be added to a plastomer in order to effect agent release [24].

It was recently discovered that pesticides and fertilizers could be incorporated in a thermoplastic matrix, and through addition o f a porosity-inducing agent, or 'porosigen', caused to emit in water or moist soil [25—27].

The porosity-inducing agent, on emission in water, creates a pore structure through which ingressing water contacts, solubilizes and removes the agent. The mechanism involved is leaching in that, unlike diffusion-dissolution, the agent does not physically move within the matrix but depends on water contact via a growing pore structure. The nature and amount o f the porosigen additives is determinant to the rate o f agent release. The polymers used are not plasticized. Technology was extended to include methods for adjusting free volume (i.e. voids between polymer molecules) to increase or decrease the size o f the developing porosity and the various geometries that the agent dispenser can take [ 2 8 - 3 0 ].

2. THERMOPLASTIC-BASED INSECTICIDE DISPENSERS



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