Pesticide microencapsulation

Microcapsule Preparation by Phase Separation Methods. The earlier methods used for pesticide microencapsulation were based on phase separation. 

Atmospheric concentrations of methyl parathion following application of the pesticide to tobacco fields were studied by Jackson and Lewis (1978). They found that levels of methyl parathion decreased rapidly following application of either the emulsifiable concentrate or the microencapsulated form. Air concentrations for the emulsifiable concentrate ranged from 7,408 ng/m immediately following application to 13 ng/m 9 days later. The corresponding measurements for the microencapsulated form were 3,783 and 16 ng/m. ... 

Microencapsulated phase-change-material (PCM) slurry, 13 276 Microencapsulation, 16 438-463. See also Encapsulation processes controlled release pesticide applications, 7 561-566... 

Scher HB, Rodson M, Lee KS. Microencapsulation of pesticides by interfacial polymerization utilizing isocyanate or aminoplast chemistry. Pest Sci 1998 54 394-400. 

Other significant uses of PCBs included heat exchangers and hydraulic fluids. Prior to controls PCBs were also used in adhesives, coatings, plasticizers and inks for microencapsulating dyes for carbonless duplicating paper as extenders in pesticide formulations and catalyst carriers in olefin polymerizations to impart hydrophobicity to materials and surfaces in bactericide formulations (combined with insecticides), and in immersion oil for microscopes. Mixed with chloronaphthalenes, PCBs were also used in wire and cable insulation in the mine and shipbuilding industries (ref. 80, p. 455). 

Microencapsulation can be used to provide a temporary barrier between a chemical species and its surrounding environment see also Section 14.3). This permits controlled (slow) release of the active agents following application. Depending on the product and the situation, an active ingredient such as a pesticide may need to be released slowly at low concentration, or slowly at high concentrations. Such controlled release can both reduce the number of crop applications that are required and also help prevent over use and subsequent run-off. The barrier can be provided by a polymer film, in the case of suspensions [867], or a liquid membrane, in the case of single or multiple emulsions [865], Microemulsions have also been used [234,865],... 

One problem in using these techniques is getting sufficient quantity of pesticide adhered to the seed or fertilizer granule. Solutions to this problem include using a powdered clay or silica to help absorb liquid pesticides, using microencapsulated liquid pesticides, and using a polymer coating to increase adhesion of pesticide particles. 

Microencapsulated Pesticides and Their Effects on Non-Target Insects... 

It has been found that not only microencapsulated methyl parathlon but also emulslflable concentrate formulations of various pesticides are carried back to hives by bees approximately In proportion to their rates of application. Field studies Involving blooming crops have demonstrated that In many Instances the microencapsulated methyl parathlon Is less hazardous to bees than the corresponding emulslflable concentrate. This hazard can be further reduced by the addition of certain adjuvants of the "sticker" type which in experimental studies has resulted In 50 to 90 percent reductions in bee mortality. Numerous field observations on various crops have shown that microencapsulated methyl parathlon frequently has a minimal adverse effect on entomophagous Insects. 

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. 

Another area of concern is the residual life of a pesticide once it is brought back to the hive. Microencapsulated methyl parathion was at one time believed to represent a special hazard because of its controlled release feature. Thus methyl parathion from MMP was reported to persist in stored pollen for up to 17 months.( ) Unfortunately, little is known about the persistence of insecticides in honey bee combs etnd the subsequent effects of their residues on the honey bees. Carbaryl has been shown to persist for at least eight months in colonies ( ) euid permethrln for at least seven months.O) Recently, USDA researchers at the University of Wisconsin studied samples from two bee kills that apparently Involved methomyl and MMP applied to sweet com. Samples were collected to determine, among others, whether methomyl persisted in combs. Analysis demonstrated that eight months after the insecticide application, residues of 0.03 ppm of methyl parathion and 0.03 ppm of methomyl ( 5) remained, even though the latter is considered to be a short-residual pesticide. 

Clearly the above data indicate that microencapsulated pesticides do not necessarily represent a unique hazard to bees, but, to the contrary, there is evidence that this hazard, at least in the case of methyl parathlon, is less than for the emulsifiable concentrate on many crops. 

Scher ( ) has reviewed the microencapsulation of pesticides and discussed some of the possible ways of achieving such a delivery system. 

Pesticides applied indoors vaporize from treated surfaces (e.g. carpets and baseboards) and can be resuspended into air on particles. Many pesticides are semivolatile (saturation vapor pressures between 10 kPa and 10 kPa at 25 °C) and tend to vaporize from treated indoor surfaces. The rate of volatilization will depend on the vapor pressure of the compound, the formulation (solvent, surfactants, microencapsulation, etc.), the ambient and surface temperatures, indoor air movement and exchange rates (ventilation), the type of surface treated and the elapsed time after application. The vapor pressure data for pure pesticides is frequently available and may be of value for assessing the relative importance... 

The rate of volatilization of microencapsulated pesticides is much slower than that of emulsifiable concentrates (Jackson and Lewis, 1979 Koehler and... 

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. 

Two types of microencapsulation are known in the art based upon the shellwall forming chemistry. These are interfacial polymerization and in-situ polymerization. Encapsulating plastic shellwalls are synthesized at the 0/W (Oil-in-Water) interface of a pesticide emulsion by reacting oil-soluble monomers dissolved in the pesticide with water-soluble monomers added to the emulsion. This process is referred to as interfacial polymerization. 

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]. 

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. 

In an alternate process organic pesticide was microencapsulated in aminoplast shellwalls starting with the urea-formaldehyde or melamine-formaldehyde prepolymers dissolved in the aqueous phase [16]. Growing polymer chains wrap around the pesticide emulsion... 

See methyl parathion. Virtually all pesticides are toxic to humans to some degree. They vary in biode-gradability. The use of more- toxic types, especially DDT, has been restricted. Microencapsulated con-trolled-release forms are available. 

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. 

Choi, K. Y. Min, K. S. Chang, T Microencapsulation of pesticides by interfacial polymerization. 2. Polyamide microcapsnles containing water-soluble drug. Polymer (1991), 15(5), 548-55. 

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