Pesticides, mineral carriers

Pesticide formulators rarely, if ever, require an unusual specification for a mineral raw material. Shortly following the rise in formulation of synthetic pesticides during and immediately after World War II the industry settled on a set of readily available, relatively inexpensive mineral carriers and solid diluents. This set has been used with little modification for some fifty years. 

Chemical Stability - The most critical test for an end-use product is chemical stability of the pesticide active ingredient. Evaluation of chemical stability does not depend on any particular property of a mineral carrier or solid diluent. Evaluations are carried out by asking whether active ingredient is stable in the end-use formulation for a certain length of time under certain storage conditions. Answering the question makes use only of a chemical analysis for active ingredient, and is essentially independent of any property of the mineral. 

Surface-catalyzed degradation of pesticides has been examined in the context of research on contaminant-clay interactions. Such interactions were observed initially when clay minerals were used as carriers and diluents in the crop protection industry (Fowker et al. 1960). Later specific studies on the persistence of potential organic contaminants in the subsurface defined the mechanism of clay-induced transformation of organophosphate insecticides (Saltzman et al. 1974 Mingelgrin and Saltzman 1977) and s-triazine herbicides (Brown and White 1969). In both cases, contaminant degradation was attributed to the surface acidity of clay minerals, controlled by the hydration status of the system. 

It has a good compatibility with all the other pesticides and with fertilisers, but certain mineral substances used as carriers cause catalytic dehydrochlorination. This can be avoided by the use of inhibitors, for example urea and hexamethylenetetramine. 

Recent patents describe the use of solvents to improve the properties of agricultural chemicals. In one invention, a carrier was developed from an agglomerated composition of plant fibers and mineral filler. The purpose of the carrier is to absorb and hold a large quantity of pesticide until it is delivered to the application site. The pesticide must be in a form of low melting liquid. In order to reduce its melting point, solvents selected from aromatic hydrocarbons are used to dissolve pesticide. In a water dispersible composition of insecticide, solvent is used to convert insecticide to a liquid form at room temperature. Solvents proposed for this application are from a group of aUcyl aromatic hydrocarbons, methyl esters of alkanoic acids, and ester mixtures derived from plant oils. 

Most pesticides are sprayed or dusted as dispersions in an inert carrier. Spraying is almost invariably in the form of an aqueous solution and in the case of dusting the carrier is a finely ground cheap mineral of some kind. Direct application of the pesticide granules to the soil for subsequent root absorption is sometimes used, and application to the whole plant in gaseous or smoke form is also occasionally employed. Seed treatment is another technique which has met with some success [22]. 

Coformulants are defined as any material other than the active ingredient which is added to a pesticidal active ingredient prior to packaging for sale to an end-user. This category includes all the items commonly employed by formulations chemists - solvents, solid diluents and carriers, surfactants, dyes, and aerosol propellants. Minerals in agricultural chemical applications will invariably be used as coformulants and not as adjuvants. 

There are two primary reasons for using any mineral in a pesticide product formulation, and a limited number of subsidiary reasons. The two overwhelming uses of minerals are as a carrier or as a solid diluent. Regardless of the means by which the end-use product is applied to a target site by a grower or commercial applicator, products utilizing minerals as carrier or solid diluent will be sold in a dry form. 

Solid Active Ingredients - Some pesticide applications require a particular physical form, generally to conform with application equipment available to the grower. Granules, for example, are commonly used as the means to apply insecticides to com. A solid active ingredient can be dissolved in a solvent and then absorbed by a carrier. This is, however, often not economical. The alternative used by the pesticide industry is to use a mineral as a substrate onto which the solid pesticide is adhered. High absorbency here is not required, and in fact can be detrimental. 

Formulations applied as sprays include wettable powders, suspension concentrates, emulsifiable concentrates, encapsulated formulations and ultra-low-volume formulations. Wettable powders consist of finely divided pesticide particles combined with a finely ground dry carrier (e.g., synthetic silica, mineral clay) and surfactants (6). When the powder is mixed with water, a stable homogeneous particle suspension is formed. Suspension concentrates are particulate insecticides premixed with a liquid. When sprayed onto porous media, the water in particle suspensions penetrates the medium leaving the insecticide at the surface. On nonporous surfaces, the water evaporates leaving a deposit. 

These minerals are used for a variety of purposes—building, whitening, manufacture of lime, portland and natural cements, fertiliser, soft glass, refractory material (dolomite), as a flux in various metallurgical processes, source of CO2, railway ballast, macadam in lithography, cement concrete and asphalt concrete, in agriculture, as a solid diluent carrier in pesticides (dolomite). 

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