Effects of Polymer Composition and Viscosity on Droplet Size of Recirculated Spray Solutions

Twelve polymers used in commercial drift retardants were tested to determine shearing effect on physical properties and sprayed droplet size distributions. Shear was applied with a shear test stand which simulated agricultural sprayers. Polymers included polyethylene oxides, polyacrylamides, and a polysaccharide, with a range of molecular weights, anionicities and concentrations. Samples of solutions containing distilled water and polymer were taken after 0, 1·0, 2·3, 3·9, 6·4 and 11·4 passes through the test stand. Size distributions of samples sprayed through a flat fan nozzle were measured. Droplet sizes of all samples decreased as recirculation increased. Increasing concentration and molecular weight increased droplet sizes of unsheared spray samples for non-ionic and anionic polymers. After 2·3 recirculations, non-ionic polymer-solutions with different concentrations and molecular weights showed DrV0·5 values little greater than water alone. Shearing also reduced DrV0·5 of anionic polymer-solutions, but even after maximum shearing, DrV0·5 remained greater than water spray. Extensional viscosity (rr2=0·78) and screen factor, a simple method of estimating extensional viscosity, (rr2=0·72) were well correlated with spray DrV0·5 values for all polymer samples. Most polymer based drift retardants lose effectiveness after recirculation through agricultural sprayers: these results showed that degradation varies greatly with polymer type.