Submitted to: Journal of Agricultural Engineering Research
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/20/1996
Publication Date: N/A
Citation: Interpretive Summary: Drift of sprayed pesticides from treated cropland onto adjoining properties can be reduced if spray is applied in larger droplets. "Drift retardants" are formulated polymers that are added to water-based spray mixtures to increase the diameter of spray drops and reduce drift. As these polymers recirculate through a sprayer system, they lose their ability to increase spray droplet size. This study measured the effect of recirculation through a shear system on the drop sizes of spray mixtures containing 12 polymers with a range of chemistry, molecular weights, anionicity, and concentrations. Extensional viscosity was measured for all mixtures. All polymers tested lost some of their ability to increase drop size after being sheared by recirculating through a centrifugal pump. After being sheared, non-ionic polymers only increased sprayed drop sizes slightly above pure water. Even increasing the concentration of a non-ionic polymer did not improve its performance by much. Anionic polymers also had decreased effect after shearing, but did produce droplets somewhat greater than water. Increasing concentration of a test anionic polymer did improve its ability to maintain increased drop sizes after being sheared. There was a high correlation between extensional viscosity and the drop sizes of sprayed mixtures. These results show great differences among non-ionic and anionic polymers to reduce drifting drops in agricultural spray applications. Using these results, chemical companies now have a powerful tool in planning new drift retardant formulations. They can measure extensional viscosity to select the best polymers. Farmers can use these results to choose the best drift retardants for use in their sprayers to protect neighbors from drift.
Technical Abstract: Twelve polymers which are representative of primary active ingredients in commercial drift retardants were tested to determine the effect of shear on droplet size, extensional viscosity and high shear rate viscosity of spray solution recirculated through a shear test stand. The polymers included poly(ethylene oxide)s (PEO), polyacrylamides (PAM), and a polysaccharide, with a range of molecular weights, anionicities and concentrations. Samples of the solutions containing distilled water and a polymer were taken after 0, 1.0, 2.3, 3.9, 6.4, and 11.4 passes through a shear test stand. Droplet size distributions of samples sprayed through an XR8004VH flat fan nozzle were measured with a Phase/Doppler Particle Analyzer. Droplet sizes of all polymer spray samples decreased as number of recirculations increased. Increasing concentration and molecular weight increased droplet sizes of unsheared spray samples for both non- ionic and anionic polymers. After being sheared, non-ionic polymer- solutions with different concentrations and molecular weights did not increase volume median diameter (VMD) much over values for water alone, while anionic polymer-solutions provided greater VMD than a water spray. An anionic polymer with anionicity of 20 mole% had the greatest VMD among all tested polymers. The combination of apparent extensional viscosity and viscosity at shear rates beyond 10,000 1/s (r-squared = 0.82) and screen factor (r-squared = 0.72) were well correlated with spray VMD, while viscosity at shear rate of 73 1/s had very low correlation (r-squared = 0.37) with VMD.