Pesticides suspension concentrates

Influence of Bentonite on the Rheology of Aqueous Pesticide Suspension Concentrates... 

The influence of addition of sodium bentonite (a commonly used antisettling system) on the rheological behaviour of a pesticide suspension concentrate (250 g dm ) has been investigated. Steady state shear stress-shear rate curves were carried out to obtain the yield value and viscosity as a function of shear rate. The shear modulus was also measured using a pulse shearometer, and the residual viscosity was obtained in afew cases from creep measurements. The rheological parameters Tg (Bingham yield value),... 

For full evaluation of the flow behaviour (rheology) of structured pesticide suspension concentrates and their settling characteristics, it is necessary to carry out measurements at small and large deformations. Such investigations provide valuable information on the viscoelastic properties of the suspension and if sufficiently analysed may be... 

Th. E. Tadros, Control and Assessment of the Physical Stability of Pesticidal Suspension Concentrates , Chem. Ind. (London), 1980, 211. 

Agricultural products - Gelling-grade clays are used to stabihze fertihzer suspensions, animal feed suspensions, and pesticide suspension concentrates. They are preferred to smectites in these applications because of their tolerance of and compatibility with high solute levels. Absorbent grades are used as pesticide carriers for dry applications and as binders and nutrient carriers in animal feeds. 

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. 

The major classes of pesticides in use in the Region are organochlo-rine and organophosphorus compounds, Ccirbamates, pyrethroids and bacterial larvicides. Organophosphorus compounds are the most common, followed by pyrethroids. Insecticides are available in a variety of formulations, including emulsifiable concentrates (EC), wet-table powders (WP), dustable powders (DP), suspension concentrates (SC), oil-in-water emulsions (EW) and capsule suspensions (CS). 

Tg and . This is shown in Fig. 7 both for the extrapolated (Bingham) and Casson s yield value. In both cases the linear relationship is maintained indicating that such crude models may be applied to the rheology of the complex system of bentonite clay plus pesticide suspension. It should be mentioned, however, that the elastic floe model is a more realistic description of the system, since the assumption of a maximum of doublets in the floe rupture model is not justified with a concentrated suspension with many body interactions. 

The sedimentation results obtained with the structured suspensions, are consistent with the predictions from rheological investigations. In the absence of any bentonite clay, the pesticidal suspension exhibits Newtonian behaviour with unmeasurable yield value, modulus or residual viscosity. In this case the particles are free to settle individually under gravity forming a dilatant sediment or clay. On the other hand, at bentonite concentrations above the gel point (> 30 g dm the non-Newtonian behaviour of the suspensions and in particular their viscoelastic behaviour results from the formation of a "three-dimensional" network, which elastically supports the total mass. After 21 weeks standing in 100 ml measuring cylinders, no separation was observed when the bentonite concentration was >37.5 g dm corresponding to a modulus > 60 Nm. Clearly the modulus value required to support the mass of the suspension depends on the density difference between particle and medium. 

Microparticles. Size matters release rates depend on surface area, ie, a function of the square of the radius of a spherical particle, and thus larger particles release for longer and are able to manipulate the external availability of the pesticide. Small microparticles are therefore limited in their scope for controlling release but can be used in traditional spraying of dispersions onto soils and crops as well as for seed dressing. Suspension concentrate formulations of matrix microparticles have been developed based on various rosins, phenoUc resins, waxes, and bitumens. These have focused on lipophilic pesticides such as trifluralin and chlorpyrifos and reductions in volatility have been demonstrated (43). 

Pesticide Dispersants. Modified lignosulfates and lignosulfonates derived from kraft lignins are used in the formulation of pesticides. In wettable powders, suspension concentrates, and water-dispersible granules, they act as dispersants and prevent sedimentation. They also act as binders in the production of granular pesticides. Typical usage levels in these types of products range from 2 to 10%. 

If the pesticide constituent is a solid that is not soluble in oil or water, dien either a suspension concentrate (SC) or a water dispersible granule (WG, WDG) can be evaluated. A suspension concentrate is a dispersion of active ingredient in a continuous aqueous phase. The concentrated suspension is then diluted in the spray tank prior to application. If the solid has to be dispersed in an oil phase, then the oil flowable (OD) has to spontaneously enuilsify if it is to be diluted into an aqueous spray tank. 

Structured Surfactant Formulations (SSF) are close-packed three-dimensional matrices of a liquid crystalline phase that suspend insoluble pesticide materials. The active ingredient can be either solid or liquid. Additional actives and/or adjuvants can be dissolved or suspended in the formulation if desired. The main advantage of an SSF over standard suspension concentrates is that no thickening or suspending agents are required. The SSF formulation can be solvent-fiee if desired, or include an oil adjuvant built in for inqiroved efficacy. 

Combination products are becoming more inportant, and adjuvants can be formulated into the pesticide concentrate as well. The result can be sophisticated products like suspomicroemulsions. The structured surfactant formulation can be evaluated in lieu of the suspension concentrate. The SSF s main advantages are the absence of thickener, and technology that allows an adjuvant to be built into die pesticide formulation. 

Polymer uses other than polymeric or slow-release pesticides have also been developed. Polymers have been used for the control of flocculation of traditional pesticide suspension formulations. This is particularly true in the preparation of concentrated suspensions, those in excess of 0.4 volume fractions of the solid.Surface active agents, of the ionic or nonionic types, can be used, but they suffer for various reasons and are not able to prevent flocculation under all conditions. 

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. 

Agricultural (Ag) formulations that are commonly diluted and applied by means of spray equipment include water soluble liquids, emulsifiable concentrates, wettable powders, and flowable suspensions. The choice of which formulation to develop normally depends upon the solubility properties of the technical pesticide. Scientists often must also consider manufacturing costs, field efficacy and product toxicity. 

Fig. 12.18A shows the results of an experiment using " C-labeled paraquat adsorbed on a clay mineral (Li-montmorillonite) suspension through a soil column. When the suspension medium was distilled water, 50% of the pesticides penetrated beyond 12 cm. Under these conditions, clay remains dispersed and pestieide is readily transported through the soil. However, for a suspension medium with an electrolyte concentration of 1 mM CaCl, paraquat remains in the upper 1 cm layer. The high calcium concentration results in rapid immobilization of the clay in the soil through flocculation, and consequently little pesticide transport occurs. 

Sorption has been commonly described as an equilibrium process, in which the pesticide molecules are rapidly and readily exchanged between the sediment and aqueous phases. In this approach ( ), the equilibrium water phase concentration, (expressed relative to suspension volume) is related to the sediment phase concentration, (expressed relative to dry weight sediment), through... 

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],... 

Muneer et al. [129] examined the photocatalytic oxidation of three pesticide derivatives, propham, propachlor and tebuthiuron in aqueous TiC>2 suspensions. The rates of degradation of each compound were found to be strongly affected by the type of TiC>2 used, pH, catalyst and substrate concentration. For each compound several intermediate products were identified using GCMS. This study indicated that the photocatalytic oxidation process proceeded by reactions involving electron transfer, hydroxyl radical and superoxide radical anions. Scheme 2 displays the proposed mechanism for the photocatalytic decomposition of propham. 

The emulsion technique was applied in the following manner Microliter quantities of the emulsifiable concentrates of the insecticides were added to distilled water. Portions of each suspension were diluted, and the concentrations of the insecticides in the final suspension were determined by electron capture gas chromatography. Appropriate amounts of the diluted emulsion suspensions were then added to about 1-liter samples of distilled water to provide concentrations of these pesticides at levels of less than one part per billion. 

The increase in gel strength with increase in bentonite concentration above the gel point is consistent with the increase in yield value and modulus. On the other hand, the limited creep measurements carried out on the present suspension showed a high residual viscosity Oq of the order of 9000 Nm s when the bentonite concentration was 45g dm. As recently pointed out by Buscall et al (27) the settling rate in concentrated suspensions depends on 0. With a model system of polystyrene latex (of radius 1.55 vim and density 1.05 g cm ) which was thickened with ethyl hydroxy ethyl cellulose, a zero shear viscosity of lONm was considered to be sufficient to reduce settling of the suspension with = 0.05. The present pesticide system thickened with bentonite gave values that are fairly high and therefore no settling was observed. 

Some pesticides are so insoluble in the solvents used in ECs that they have to be formulated in other ways. In this case, these pesticides can be formulated to become water-based mixtures that can be handled and applied in the same manner as ECs. Basically, suspend-able concentrate formulations that contain 50% to 90% of the toxicant are WPs of small particle size (1-5 pm) that remain in suspension for long periods. Suspendability and storage stability are improved by inclusion of surfactants and various additives. Oils can be added when penetration of plants is desired. 

---