Spray atomization characteristics as a function of pesticide formulations and atomizer design

Abstract

The use of formulation adjuvants to increase the drop size of pesticide sprays has followed a practice which has evolved through many years of experimentation and development. The earliest materials used were simply designed to make the spray mix viscous on the premise that a mayonnaise‐like fluid would produce larger drops and hence increase the deposit efficiency. These viscomer materials, emulsions and water soluble thickeners such as starch and agars, as well as more complex cellulose materials, produced thick non‐Newtonian fluids which were difficult to mix, pump and spray, and provided questionable results. First, the larger drops reduced target coverage from a given volume of spray; and second, field tests with these definitely showed that a large volume of small drops were still being produced even with the thickest of formulations.

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 bonding, they have the capability of forming long string‐like streams which when atomized can retract into a spray of large drops. Pure forms of these polymers are blended by commercial producers to enable compatibility with pesticide chemicals. We have conducted a series of laboratory 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 drift‐prone drops (i.e., those smaller than 120 μm in diameter).

Our results with the laboratory studies of physical properties and of the wind tunnel drop size tests have been inconclusive. The elastomer materials have relatively low viscosity (1.5 to 6 mPa.s) and reduced surface 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 increased for all of the fan (deflector and milled orifice) and cone type atomizers. But an increase was also indicated of the volume of spray in drops less than 120 μm showing that the polymer did not fully control the production of these small drops under all of the testing procedures we studied.

It is difficult to evaluate the capability of these additives for controlling or reducing production of the drift‐prone small drops. While the pure solutions of the water soluble polymers would appear to be capable of this desirable effect, it is also evident that in order to make these formulations compatible with pesticide spray solutions, several other solvents, emulsifiers and surface active chemicals are added to the formulation, thus affecting the atomization characteristics and hence their drift‐control capability.     

Optimising herbicide dose: a straightforward approach to reduce the risk of side effects of herbicides

Public concern of possible effects of pesticides on human health and the environment has lead to an increased pressure on farmers to optimise their use of pesticides. Reducing pesticide doses below the recommended doses whenever possible is a straightforward approach to reduce the risk of adverse side effects. To adopt this approach decision-making has to be improved. The parameters to consider optimising herbicide doses are: weed flora and growth stage, crop competitiveness, climatic conditions, application technique, formulation/adjuvant and combination with other pesticides. In Denmark this information is provided to farmers through the decision support system ‹Plant Protection Online’. Based on input on weed species and densities, climatic conditions, soil type, crop cultivar and expected yield ‹Plant Protection Online’ will provide farmers with optimum herbicide solution usually with doses lower than standard recommended doses.