Pesticide microcapsules

In another report, Scher discussed pesticide microcapsules and reported that these capsules could be used to reduce mammalian toxicity, to extend activity, to reduce phytotoxicity, to protect pesticides from rapid environmental degradation, and to reduce pesticide levels in the environment [68]. Thus, many advantages are obtainable with microcapsular pesticide formulations. In an overview, Gimeno has discussed the definition of the criteria to select an active agent for microencapsulation, together with the techniques used in commercial microencapsulated formulations [69]. 

Preparation and scanning electron microscopy of microcapsules of Abate, Pesticides. 9-11, 1986. 

Select the active ingredient (AI) or drug/pesticide product and the microcapsule encapsulating it by evaluating the controlled release of the AI [Muro-Sune et al. (2005)] through the microcapsule. 

Consider the following product design and evaluation problem from the agrochemical industry. A pesticide product consisting of an active ingredient and an additive need to be evaluated in terms of its controlled release characteristics from a polymeric microcapsule to a release medium. Here, since the AI and the additives (solvent and surfactant) are known, the product-evaluation problem consists of designing the microcapsule and identification of... 

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. 

The widely used organophosphate Insecticide methyl parathlon was the first material to be formulated as a microencapsulated pesticide. This formulation, sold under the tradename PENNCAP-M Insecticide (a registered trademark of Pennwalt Corporation), consists of nylon-type microcapsules which contain the active Ingredient. The capsules are suspended In water and typically have an average particle size of approximately 25 microns (fifty percent by weight of the capsules have a particle size of 25 microns or more). Upon application by conventional spray equipment the water evaporates, and the active Ingredient Is slowly released over an extended period of time. 

By virtue of their unique formulation, microencapsulated pesticides have significantly lower dermal and oral toxiclties than the corresponding emulslflable concentrate formulations. Microencapsulated methyl parathion (MMP) was Introduced commercially in 1974 In 1976 and thereafter reports of bee kills allegedly caused by this product were published, primarily In the news media and non-refereed journals. Claims were subsequently made that MMP Is carried back to the hive and Is uniquely hazardous because the size of the microcapsules Is In the range of many pollens. 

Controlled release formulations are a recent innovation in which the pesticide is incorporated into a carrier, generally a polymeric material (Scher, 1999). The rate of release of the pesticide is determined by the properties of the polymer itself as well as environmental factors. There are mainly two types of CR formulations reservoir devices and monolithic devices. As shown in Figure 2.1, in the reservoir device, the toxicant is enclosed in capsules of thin polymeric material to become microcapsules (1-100 pm in diameter), e.g., Penncap-M microcapsules (methyl parathion). In the monolithic device, the toxicant is uniformly... 

Microcapsules are made by a two-step process mixing two chemical systems, the pesticide in a solvent, and the capsule material, so that a wall forms around pesticide droplets (Figure 2.2). The size of the capsule can be controlled by mixing speed, by the chemical used, and by the conditions. The capsule material used will determine the nature of the wall, such as porosity and decomposition (Terriere, 1982). 

Figure 2.2 Electron micrograph of microcapsules on filter paper (lOOOx). (From Morgan, R.L., et al., in Pesticide Formulations Innovations and Developments, Cross, B. and Scher, H.B., Eds., ACS Symposium Series 371, American Chemical Society, Washington, D.C., 1988, p. 131. With permission.)...

A milestone in application of microencapsulation to herbicides was the development of high concentration formulations. Commercial products containing 480 grams of herbicide in microcapsules per liter volume of formulation were possible. This development was based upon the discovery of dispersing polymers that could emulsify a larger volume of pesticide into a smaller volume of water [4]. 

Recent adaptations of this technology provide structurally sound shellwalls that can be varied to release in a controlled manner [5]. Conventional methods to alter release fh)m microcapsules include using less monomer and making smaller microcapsules. However, processing in this way to control shellwall thickness could lead to fragile microcapsules. Controlled release with more robust microcapsules was achieved by using specified ratios of unique diisocyanate and triisocyanate monomers dissolved in the pesticide. Diamine monomers added to the emulsion in the typical manner produced robust microcapsules with desired rates of release. 

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]. 

Another innovation in in-situ microencapsulation is aminoplast shellwalls containing base-cleavable ester moiety [15]. Polyols reacted with diacids that contain thiol or hydroxy functionality produce crosslinking groups. These crosslinking groups along with urea-formaldehyde prepolymer are dissolved into the pesticide and the in-situ microencapsulation process is completed. The resultant microcapsules may contain an insecticide that is safer to handle under acidic conditions yet will rapidly release the insecticide in the alkaline gut of an insect. Alternately, base could be added to the spray tank to rapidly release capsule contents prior to application. 

Currently, the release process from microcapsule formulations is managed by simple diffusion of the active ingredient through the polymer walls. Generally, an increase in the pesticide breakdown brought about by rising soil temperatures is partially compensated by an increase in the rate of diffusion of the pesticide through the microcapsule walls. 

Microcapsules with permeable walls enable prolonged release of the active components into the environment, such as in the case of prolonged release drugs, perfumes, deodorants, repellents, etc., or immobilization with locally limited activity of micro-encapsulated substances. Examples of the latter include microencapsulated fertilizers and pesticides with locally limited release to reduce leaching into the ground water, or microencapsulated catalysts and enzymes for chemical and biotechnological processes. 

Hirech et al. have described preparation of polyurea microcapsules containing hquid pesticide (e.g., diazinon) suspended in concentrated disinfectant solution, by use of a two-step microencapsulation process [28], The first step is the hquid-hquid dispersion in a Sulzer static mixer (SMX) the second step is microencapsulation by inter-fadal polymerization in a stirred-tank reactor (Fig. 5.4). It has been shown that the... 

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. 

Such a product would bait or lure a pest to a capsule, but release the pesticide only when the pest digests or attacks the capsule. Using this method, UF microcapsules containing pesticide were prepared such that the capsule size was very small (-15 pm). The second wall material is formed by mixing appropriate amounts of gelatin, gum arabic, and ethyl cellulose in water. UF microcapsules mixed with peanut oil (as attractant) are added to the second wall material solution which, after stirring for 1 h at 65 °C, was cooled to room temperature and stirred for another hour. The resultant gelatin-based shell material encapsulated both peanut oil and UF microcapsules. 

Microcapsules with desired release rates according to the application involved can be prepared by incorporating a selected fluid (solvent), while keeping the other capsule parameters constant. These fluids include dearomatized and isoparaffinic hydrocarbons, aromatic hydrocarbons, acetate derivatives, and blends of these [51], In the preparation of polyurethane-polyurea microcapsules containing hep-tenophos (liquid pesticide, b.p. 64 °C), it has been reported that the porosity of microcapsules can be controlled by the addition of 2-ethoxy ethyl acetate or ethyl acetate in the organic phase containing isocyanate prepolymer and pesticide [52]. 

Aminoplast (MF) has been used to prepare microcapsules of hquid pesticides, namely methyl parathion [71] and metachlor [72]. Other selected examples of microcapsules containing agrochemicals such as pesticides, herbicides, fertihzers and insecticides have been reported and are summarized in Table 5.10. 

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