SPRAY ATOMIZATION CHARACTERISTICS AS A FUNCTION OF PESTICIDE FORMULATIONS AND ATOMIZER DESIGN

The use of formulation adjuvants to increase the drop size of pesticid e sprays has followed a practice which has evolved through many years of experimentation and development. The earliest materials used were s imply designed to make the spray mix viscous on the premise that a may onnaise-like fluid would produce larger drops and hence increase the d eposit efficiency. These viscomer materials, emulsions and water solub le thickeners such as starch and agars, as well as more complex cellul ose materials, produced thick non-Newtonian fluids which were difficul t to mix, pump and spray, and provided questionable results. First, th e larger drops reduced target coverage from a given volume of spray; a nd second, field tests with these definitely showed that a large volum e of small drops were still being produced even with the thickest of f ormulations. The next step in adjuvant evolution was the introduction of polyvinyl, polyacrylamide and polyamide elastomer materials. These polymers are also non-Newtonian, but due to their peculiar molecular b onding, they have the capability of forming long string-like streams w hich when atomized can retract into a spray of large drops. Pure forms of these polymers are blended by commercial producers to enable compa tibility with pesticide chemicals. We have conducted a series of labor atory and wind tunnel tests; first, to try and establish some physical parameters identifying their characteristics and second, to determine how these adjuvants affect atomization and the production of small dr ift-prone drops (i.e., those smaller than 120 mu m in diameter). Our r esults with the laboratory studies of physical properties and of the w ind tunnel drop size tests have been inconclusive. The elastomer mater ials have relatively low viscosity (1.5 to 6 mPa.s) and reduced surfac e tension (50 to 60 mN/m) but our stream-flow extension tests were non -productive. The drop size studies comparing water sprays with polymer -water mixtures gave us a mixed picture. Generally drop size was incre ased for all of the fan (deflector and milled orifice) and cone type a tomizers. But an increase was also indicated of the volume of spray in drops less than 120 mu m showing that the polymer did not fully contr ol the production of these small drops under all of the testing proced ures we studied. It is difficult to evaluate the capability of these a dditives for controlling or reducing production of the drift-prone sma ll drops. While the pure solutions of the water soluble polymers would appear to be capable of this desirable effect, it is also evident tha t in order to make these formulations compatible with pesticide spray solutions, several other solvents, emulsifiers and surface active chem icals are added to the formulation, thus affecting the atomization cha racteristics and hence their drift-control capability.