Encapsulation of pesticide

Shasha, B.S., Trimnell, D. and Otey, F.H. (1981) Encapsulation of pesticides in a starch-calcium adduct. Joumol of Polymer Science Polymer Chemistry Edition, 19,... 

The main purpose of pesticide formulation is to manufacture a product that has optimum biological efficiency, is convenient to use, and minimizes environmental impacts. The active ingredients are mixed with solvents, adjuvants (boosters), and fillers as necessary to achieve the desired formulation. The types of formulations include wettable powders, soluble concentrates, emulsion concentrates, oil-in-water emulsions, suspension concentrates, suspoemulsions, water-dispersible granules, dry granules, and controlled release, in which the active ingredient is released into the environment from a polymeric carrier, binder, absorbent, or encapsulant at a slow and effective rate. The formulation steps may generate air emissions, liquid effluents, and solid wastes. 

It is also possible to generate microcapsules through interfacial polymerization using only one monomer to form the shell. In this class of encapsulations, polymerization must be performed with a surface-active catalyst, a temperature increase, or some other surface chemistry. Herbert Scher of Zeneca Ag Products (formerly Stauffer Chemical Company) developed an excellent example of the latter class of shell formation (Scher 1981 Scher et al. 1998). He used monomers featuring isocyanate groups, like poly(methylene)-poly(phenylisocyanate) (PMPPI), where the isocyanate reacts with water to reveal a free primary amine. Dissolved in the oil-dispersed phase of an oil-in-water emulsion, this monomer contacts water only at the phase boundary. The primary amine can then react with isocyanates to form a polyurea shell. Scher used this technique to encapsulate pesticides, which in their free state would be too volatile or toxic, and to control the rate of pesticide release. 

The tnicroencapsululion of pesticides (qv) and herbicides active area of development that has produced several commercial products. The function of the microcapsules is to prolong activity while reducing mammalian toxicity, volatilization losses, phytotoxicity. environmental degradation, and movement in the soil. Ideally, encapsulation would also reduce the amount of agrochemical needed. 

Encapsulation is a process in which tiny particles or droplets are covered by a coating [36-43]. Its role is either to isolate the active ingredient or to control the rate by which it leaves the capsule. As examples for the first case, one can mention the isolation of vitamins from oxygen or of a reactive core from chemical attack, and for the second case, the control of the rate of release of drugs or pesticides. Numerous encapsulation techniques have been suggested. Most of the encapsulations of the active solid materials have been carried out for solids larger than 1 pm. 

Two distinct controlled release technologies are encapsulation of liquid pesticides and the coating of individual pesticide crystals. Encapsulation of liquid pesticides is an established tool for modem formulators. Commercial microencapsulated pesticide products exist and new developments continue to be made. Coating of individual pesticide crystals without their aggregation is more difficult. While new processes do exist to coat pesticide crystals without aggregation these processes have not yet been utilized to create commercial pesticide products. 

A recent innovation in in-situ microencapsulation is the development of acid-triggered release of pesticide from the microcapsules [12]. Diols and aldehydes are reacted to form an acid labile acetal moiety. The acetal is then reacted with isocyanate to create a prepolymer. The prepolymer is a polyisocyanate cmitaining the acid labile moiety and suitable for in-situ shellwall polymerization. The prepolymer is dissolved into a pesticide, emulsified into water, and shellwall formed in-situ. Under alkaline or neutral pH conditions in a container, the insecticide is safely contained in the microcapsules. Acid could be added to the spray tank to rapidly release capsule contents prior to application. Alternate shellwall chemistry for in-situ microencapsulation utilizes etherified urea-formaldehyde prepolymers in the oil phase that are self-condensed with acid catalyst to produce encapsulating aminoplast shellwalls [13]. This process does not have the problem of continuing CO2 evolution. Water-soluble urea-formaldehyde and melamine-formaldehyde prepolymers can be selected to microencapsulate water or aqueous solutions [14]. 

Xanthates were also used for microencapsulation of pesticides the pesticide and a soluble xanthate were blended in aqueous solution followed by acidification and the addition of a coupling agent to form a matrix.2218-2227 Particles of nitrile-butadiene rubber (NBR) and styrene-butadiene rubber (SBR) were also encapsulated by starch xanthates.2228... 

Microencapsulation is also of importance in agricultural applications. Encapsulation of fertilizers ensures their slow release into soil and more uniform feeding to plants. Microencapsulation allows one to coat the surface of seeds with various important substances, such as growth agents, pesticides, and fertilizers. 

Microcapsules containing liquid pesticide have certain drawbacks. One example is when the pesticide is itself both volatile and toxic and has a high vapor pressure. A second example is when the capsule shell is strong and thick. In the first case, the pesticide diffuses very rapidly from the capsules and its odor initially repels the pest. Diffusion from the capsules is rapid, however, and when they are empty the pests return to the site (e.g., crops). In the second case, the capsules do not release the pesticide to produce a minimum effective level at the application site, and so pestiddal action is not achieved. In order to overcome these problems, a WO patent disclosed the preparation of microcapsules of pesticides containing pest attractant using a capsule-in-capsule approach [50]. As shown in Figure 5.18, the outer capsule contains pest attractant or food, in which the iruier capsule containing the pesticide, is encapsulated. 

Phase separation can also be produced from solutions of polymers in organic solvents. By addition of a nonsolvent for the polymer to the solution containing the core material the pol)mier will precipitate aroimd the emulsified core to form microcapsules (30). This can allow for the encapsulation of aqueous solutions or suspensions of pesticides. For example, such an aqueous solution can be emulsified in oil containing the dissolved polymer. Addition of the nonsolvent to the oil phase separates out the polymer, which can then form the wall around the water droplets. 

In most cases a test for direct material toxicity to animals (birds, rats, rabbits and so on), may not be of primary interest. Such interactions should only be considered if a product is applied in a shape and colour that may be confused with feed by such animals and may be consumed by them in relevant amounts. The encapsulation of fertilisers or pesticides with biodegradable polymers in granular shape may be such a case. On the other hand, the encapsulated substances are most probably more harmful to animals than the biodegradable polymer coating. 

The indiscriminate use of such agricultural chemicals as pesticides, herbicides, and fertilizers is an important source of environmental pollution. A novel application of polymer-bound materials has been made in the controlled release of agricultural chemicals (Allan et al., 1973 Beasley and Collins, 1970 Shambu et al., 1976 Schacht et al., 1977, 1978 reviews of Neogi and Allan, 1974 Scher, 1977). When these chemicals are covalently bound to a polymer from which they can be slowly released into the environment, they not only check pollution but their duration of action is prolonged. The same effect can be obtained by encapsulation of the chemicals in polymeric beads from which they can be released slowly, e.g., 2,4-dichlorophenoxyacetic acid and 4-amino-3,5,6-trichloropicolinic acid have been used in polymer-bound form. 

In the following discussion, pesticides that have been dispersed within the polymer in some way or encapsulated as a solution within a polymeric "skin will be discussed under the "Physically Bonded" topics. These systems are those that physically hold the pesticide within the polymeric matrix. On the other hand, the "Chemically Bonded" systems bind the pesticide or pesticidal ligand to part of the molecule of the polymer itself. Their action is due to either the action of the polymer itself or to the gradual breakdown of the molecule leaving a smaller molecule having pesticidal action. Specialized uses will be found in the sections on "Flocculation Control" and "Spray Drift Control". References that emphasize the incorporation of pesticide-polymers as "Polymeric Devices" and those that control insects and weeds through "Plastic Mulches" will be discussed last. 

An important development in Japan is the use of biodegradable polyolefins in controlled release of fertilisers by encapsulation [30]. This results in controlled release in leaching environments over an extended period of time compared with direct application. This in turn effectively reduces the pollution of streams and the eutrophication of lakes and watercourses. Controlled release of pesticides by encapsulation has also considerable potential by matching the application time to the life cycle of the pest [11]. 

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